| Message ID | 20200909152047.27905-1-zangchunxin@bytedance.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] mm/vmscan: fix infinite loop in drop_slab_node | expand |
Thanks! This looks reasonable to me, and avoids hard-to-reason-about changes in the existing semantics. zangchunxin@bytedance.com writes: >Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com> >Signed-off-by: Muchun Song <songmuchun@bytedance.com> Acked-by: Chris Down <chris@chrisdown.name>
On 9/9/20 5:20 PM, zangchunxin@bytedance.com wrote: > From: Chunxin Zang <zangchunxin@bytedance.com> > > On our server, there are about 10k memcg in one machine. They use memory > very frequently. When I tigger drop caches，the process will infinite loop > in drop_slab_node. > > There are two reasons: > 1.We have too many memcgs, even though one object freed in one memcg, the > sum of object is bigger than 10. > > 2.We spend a lot of time in traverse memcg once. So, the memcg who > traversed at the first have been freed many objects. Traverse memcg next > time, the freed count bigger than 10 again. > > We can get the following info through 'ps': > > root:~# ps -aux | grep drop > root 357956 ... R Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches > root 1771385 ... R Aug16 21146421:17 echo 3 > /proc/sys/vm/drop_caches > root 1986319 ... R 18:56 117:27 echo 3 > /proc/sys/vm/drop_caches > root 2002148 ... R Aug24 5720:39 echo 3 > /proc/sys/vm/drop_caches > root 2564666 ... R 18:59 113:58 echo 3 > /proc/sys/vm/drop_caches > root 2639347 ... R Sep03 2383:39 echo 3 > /proc/sys/vm/drop_caches > root 3904747 ... R 03:35 993:31 echo 3 > /proc/sys/vm/drop_caches > root 4016780 ... R Aug21 7882:18 echo 3 > /proc/sys/vm/drop_caches > > Use bpftrace follow 'freed' value in drop_slab_node: > > root:~# bpftrace -e 'kprobe:drop_slab_node+70 {@ret=hist(reg("bp")); }' > Attaching 1 probe... > ^B^C > > @ret: > [64, 128) 1 | | > [128, 256) 28 | | > [256, 512) 107 |@ | > [512, 1K) 298 |@@@ | > [1K, 2K) 613 |@@@@@@@ | > [2K, 4K) 4435 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| > [4K, 8K) 442 |@@@@@ | > [8K, 16K) 299 |@@@ | > [16K, 32K) 100 |@ | > [32K, 64K) 139 |@ | > [64K, 128K) 56 | | > [128K, 256K) 26 | | > [256K, 512K) 2 | | > > In the while loop, we can check whether the TASK_KILLABLE signal is set, > if so, we should break the loop. That's definitely a good change, thanks. I would just maybe consider: - Test in the memcg iteration loop? If you have 10k memcgs as you mention, this can still take long until the test happens? - Exit also on other signals such as SIGABRT, SIGTERM? If I write to drop_caches and think it's too long, I would prefer to kill it by ctrl-c and not just kill -9. Dunno if the canonical way of testing for this is if (signal_pending(current)) or differently. - IMHO it's still worth to bail out in your scenario even without a signal, e.g. by the doubling of threshold. But it can be a separate patch. Thanks! > Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com> > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > --- > changelogs in v2: > 1) Via check TASK_KILLABLE signal break loop. > > mm/vmscan.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index b6d84326bdf2..c3ed8b45d264 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -704,6 +704,9 @@ void drop_slab_node(int nid) > do { > struct mem_cgroup *memcg = NULL; > > + if (fatal_signal_pending(current)) > + return; > + > freed = 0; > memcg = mem_cgroup_iter(NULL, NULL, NULL); > do { >
Vlastimil Babka writes: >- Exit also on other signals such as SIGABRT, SIGTERM? If I write to drop_caches >and think it's too long, I would prefer to kill it by ctrl-c and not just kill Oh dear, fatal_signal_pending() doesn't consider cases with no more userspace instructions due to SIG_DFL on TERM/INT etc, that seems misleading :-( I had (naively) believed it internally checks the same set as TASK_KILLABLE. Chuxin, Muchun, can you please make it work using TASK_KILLABLE in a similar way to how schedule_timeout_killable and friends do it instead, so that other signals will be caught?
On Wed, Sep 09, 2020 at 07:59:44PM +0200, Vlastimil Babka wrote: > - Exit also on other signals such as SIGABRT, SIGTERM? If I write to drop_caches > and think it's too long, I would prefer to kill it by ctrl-c and not just kill > -9. Dunno if the canonical way of testing for this is if > (signal_pending(current)) or differently. fatal_signal_pending() is the canonical way to do it. If your task has installed a signal handler for ABRT or TERM, that's its prerogative, but it's chosen not to get killed, and it's not allowed to see short reads/writes, so we can't break out early.
On Wed, Sep 09, 2020 at 10:47:24PM +0100, Chris Down wrote: > Vlastimil Babka writes: > > - Exit also on other signals such as SIGABRT, SIGTERM? If I write to drop_caches > > and think it's too long, I would prefer to kill it by ctrl-c and not just kill > > Oh dear, fatal_signal_pending() doesn't consider cases with no more > userspace instructions due to SIG_DFL on TERM/INT etc, that seems misleading > :-( I had (naively) believed it internally checks the same set as > TASK_KILLABLE. > > Chuxin, Muchun, can you please make it work using TASK_KILLABLE in a similar > way to how schedule_timeout_killable and friends do it instead, so that > other signals will be caught? You're mistaken. if (sig_fatal(p, sig) && !(signal->flags & SIGNAL_GROUP_EXIT) && !sigismember(&t->real_blocked, sig) && (sig == SIGKILL || !p->ptrace)) { ... sigaddset(&t->pending.signal, SIGKILL); static inline int __fatal_signal_pending(struct task_struct *p) { return unlikely(sigismember(&p->pending.signal, SIGKILL)); }
Matthew Wilcox writes: >On Wed, Sep 09, 2020 at 10:47:24PM +0100, Chris Down wrote: >> Vlastimil Babka writes: >> > - Exit also on other signals such as SIGABRT, SIGTERM? If I write to drop_caches >> > and think it's too long, I would prefer to kill it by ctrl-c and not just kill >> >> Oh dear, fatal_signal_pending() doesn't consider cases with no more >> userspace instructions due to SIG_DFL on TERM/INT etc, that seems misleading >> :-( I had (naively) believed it internally checks the same set as >> TASK_KILLABLE. >> >> Chuxin, Muchun, can you please make it work using TASK_KILLABLE in a similar >> way to how schedule_timeout_killable and friends do it instead, so that >> other signals will be caught? > >You're mistaken. > > if (sig_fatal(p, sig) && > !(signal->flags & SIGNAL_GROUP_EXIT) && > !sigismember(&t->real_blocked, sig) && > (sig == SIGKILL || !p->ptrace)) { >... > sigaddset(&t->pending.signal, SIGKILL); > >static inline int __fatal_signal_pending(struct task_struct *p) >{ > return unlikely(sigismember(&p->pending.signal, SIGKILL)); >} Oh great, that makes things way easier. Thanks!
On 9/9/20 11:52 PM, Matthew Wilcox wrote: > On Wed, Sep 09, 2020 at 10:47:24PM +0100, Chris Down wrote: >> Vlastimil Babka writes: >> > - Exit also on other signals such as SIGABRT, SIGTERM? If I write to drop_caches >> > and think it's too long, I would prefer to kill it by ctrl-c and not just kill >> >> Oh dear, fatal_signal_pending() doesn't consider cases with no more >> userspace instructions due to SIG_DFL on TERM/INT etc, that seems misleading >> :-( I had (naively) believed it internally checks the same set as >> TASK_KILLABLE. >> >> Chuxin, Muchun, can you please make it work using TASK_KILLABLE in a similar >> way to how schedule_timeout_killable and friends do it instead, so that >> other signals will be caught? > > You're mistaken. Ah actually it was me who thought fatal_signal_pending() was only for SIGKILL, OOM and whatnot. Sorry for the noise. > if (sig_fatal(p, sig) && > !(signal->flags & SIGNAL_GROUP_EXIT) && > !sigismember(&t->real_blocked, sig) && > (sig == SIGKILL || !p->ptrace)) { > ... > sigaddset(&t->pending.signal, SIGKILL); > > static inline int __fatal_signal_pending(struct task_struct *p) > { > return unlikely(sigismember(&p->pending.signal, SIGKILL)); > } >
The subject is misleading because this patch doesn't fix an infinite loop, right? It just allows the userspace to interrupt the operation. On Wed 09-09-20 23:20:47, zangchunxin@bytedance.com wrote: > From: Chunxin Zang <zangchunxin@bytedance.com> > > On our server, there are about 10k memcg in one machine. They use memory > very frequently. When I tigger drop caches，the process will infinite loop > in drop_slab_node. Is this really an infinite loop, or it just takes a lot of time to process all the metadata in that setup? If this is really an infinite loop then we should look at it. My current understanding is that the operation would finish at some time it just takes painfully long to get there. > There are two reasons: > 1.We have too many memcgs, even though one object freed in one memcg, the > sum of object is bigger than 10. > > 2.We spend a lot of time in traverse memcg once. So, the memcg who > traversed at the first have been freed many objects. Traverse memcg next > time, the freed count bigger than 10 again. > > We can get the following info through 'ps': > > root:~# ps -aux | grep drop > root 357956 ... R Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches > root 1771385 ... R Aug16 21146421:17 echo 3 > /proc/sys/vm/drop_caches > root 1986319 ... R 18:56 117:27 echo 3 > /proc/sys/vm/drop_caches > root 2002148 ... R Aug24 5720:39 echo 3 > /proc/sys/vm/drop_caches > root 2564666 ... R 18:59 113:58 echo 3 > /proc/sys/vm/drop_caches > root 2639347 ... R Sep03 2383:39 echo 3 > /proc/sys/vm/drop_caches > root 3904747 ... R 03:35 993:31 echo 3 > /proc/sys/vm/drop_caches > root 4016780 ... R Aug21 7882:18 echo 3 > /proc/sys/vm/drop_caches > > Use bpftrace follow 'freed' value in drop_slab_node: > > root:~# bpftrace -e 'kprobe:drop_slab_node+70 {@ret=hist(reg("bp")); }' > Attaching 1 probe... > ^B^C > > @ret: > [64, 128) 1 | | > [128, 256) 28 | | > [256, 512) 107 |@ | > [512, 1K) 298 |@@@ | > [1K, 2K) 613 |@@@@@@@ | > [2K, 4K) 4435 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| > [4K, 8K) 442 |@@@@@ | > [8K, 16K) 299 |@@@ | > [16K, 32K) 100 |@ | > [32K, 64K) 139 |@ | > [64K, 128K) 56 | | > [128K, 256K) 26 | | > [256K, 512K) 2 | | > > In the while loop, we can check whether the TASK_KILLABLE signal is set, > if so, we should break the loop. I would make it explicit that this is not fixing the above scenario. It just helps to cancel to operation which is a good thing in general. > Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com> > Signed-off-by: Muchun Song <songmuchun@bytedance.com> With updated changelog Acked-by: Michal Hocko <mhocko@suse.com> > --- > changelogs in v2: > 1) Via check TASK_KILLABLE signal break loop. > > mm/vmscan.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index b6d84326bdf2..c3ed8b45d264 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -704,6 +704,9 @@ void drop_slab_node(int nid) > do { > struct mem_cgroup *memcg = NULL; > > + if (fatal_signal_pending(current)) > + return; > + > freed = 0; > memcg = mem_cgroup_iter(NULL, NULL, NULL); > do { > -- > 2.11.0 >
On Mon, Sep 14, 2020 at 5:30 PM Michal Hocko <mhocko@suse.com> wrote: > The subject is misleading because this patch doesn't fix an infinite > loop, right? It just allows the userspace to interrupt the operation. > > Yes, so we are making a separate patch follow Vlastimil's recommendations. Use double of threshold to end the loop. On Thu, Sep 10, 2020 at 1:59 AM Vlastimil Babka <vbabka@suse.cz> wrote: > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > ... > - IMHO it's still worth to bail out in your scenario even without a > signal, e.g. > by the doubling of threshold. But it can be a separate patch. > Thanks! > ... On Wed 09-09-20 23:20:47, zangchunxin@bytedance.com wrote: > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > On our server, there are about 10k memcg in one machine. They use memory > > very frequently. When I tigger drop caches，the process will infinite loop > > in drop_slab_node. > > Is this really an infinite loop, or it just takes a lot of time to > process all the metadata in that setup? If this is really an infinite > loop then we should look at it. My current understanding is that the > operation would finish at some time it just takes painfully long to get > there. > Yes, it's really an infinite loop. Every loop spends a lot of time. In this time, memcg will alloc/free memory, so the next loop, the total of 'freed' always bigger than 10. > > There are two reasons: > > 1.We have too many memcgs, even though one object freed in one memcg, the > > sum of object is bigger than 10. > > > > 2.We spend a lot of time in traverse memcg once. So, the memcg who > > traversed at the first have been freed many objects. Traverse memcg > next > > time, the freed count bigger than 10 again. > > > > We can get the following info through 'ps': > > > > root:~# ps -aux | grep drop > > root 357956 ... R Aug25 21119854:55 echo 3 > > /proc/sys/vm/drop_caches > > root 1771385 ... R Aug16 21146421:17 echo 3 > > /proc/sys/vm/drop_caches > > root 1986319 ... R 18:56 117:27 echo 3 > /proc/sys/vm/drop_caches > > root 2002148 ... R Aug24 5720:39 echo 3 > /proc/sys/vm/drop_caches > > root 2564666 ... R 18:59 113:58 echo 3 > /proc/sys/vm/drop_caches > > root 2639347 ... R Sep03 2383:39 echo 3 > /proc/sys/vm/drop_caches > > root 3904747 ... R 03:35 993:31 echo 3 > /proc/sys/vm/drop_caches > > root 4016780 ... R Aug21 7882:18 echo 3 > /proc/sys/vm/drop_caches > > > > Use bpftrace follow 'freed' value in drop_slab_node: > > > > root:~# bpftrace -e 'kprobe:drop_slab_node+70 {@ret=hist(reg("bp")); }' > > Attaching 1 probe... > > ^B^C > > > > @ret: > > [64, 128) 1 | > | > > [128, 256) 28 | > | > > [256, 512) 107 |@ > | > > [512, 1K) 298 |@@@ > | > > [1K, 2K) 613 |@@@@@@@ > | > > [2K, 4K) 4435 > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| > > [4K, 8K) 442 |@@@@@ > | > > [8K, 16K) 299 |@@@ > | > > [16K, 32K) 100 |@ > | > > [32K, 64K) 139 |@ > | > > [64K, 128K) 56 | > | > > [128K, 256K) 26 | > | > > [256K, 512K) 2 | > | > > > > In the while loop, we can check whether the TASK_KILLABLE signal is set, > > if so, we should break the loop. > > I would make it explicit that this is not fixing the above scenario. It > just helps to cancel to operation which is a good thing in general. > > > Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com> > > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > > With updated changelog > Acked-by: Michal Hocko <mhocko@suse.com> > > > --- > > changelogs in v2: > > 1) Via check TASK_KILLABLE signal break loop. > > > > mm/vmscan.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index b6d84326bdf2..c3ed8b45d264 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -704,6 +704,9 @@ void drop_slab_node(int nid) > > do { > > struct mem_cgroup *memcg = NULL; > > > > + if (fatal_signal_pending(current)) > > + return; > > + > > freed = 0; > > memcg = mem_cgroup_iter(NULL, NULL, NULL); > > do { > > -- > > 2.11.0 > > > > -- > Best wishes > Chunxin >
On Mon 14-09-20 21:25:59, Chunxin Zang wrote: > On Mon, Sep 14, 2020 at 5:30 PM Michal Hocko <mhocko@suse.com> wrote: > > > The subject is misleading because this patch doesn't fix an infinite > > loop, right? It just allows the userspace to interrupt the operation. > > > > > Yes, so we are making a separate patch follow Vlastimil's recommendations. > Use double of threshold to end the loop. That still means the changelog needs an update > On Thu, Sep 10, 2020 at 1:59 AM Vlastimil Babka <vbabka@suse.cz> wrote: > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > ... > > - IMHO it's still worth to bail out in your scenario even without a > > signal, e.g. > > by the doubling of threshold. But it can be a separate patch. > > Thanks! > > ... > > > > On Wed 09-09-20 23:20:47, zangchunxin@bytedance.com wrote: > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > > On our server, there are about 10k memcg in one machine. They use memory > > > very frequently. When I tigger drop caches，the process will infinite loop > > > in drop_slab_node. > > > > Is this really an infinite loop, or it just takes a lot of time to > > process all the metadata in that setup? If this is really an infinite > > loop then we should look at it. My current understanding is that the > > operation would finish at some time it just takes painfully long to get > > there. > > > > Yes, it's really an infinite loop. Every loop spends a lot of time. In > this time, > memcg will alloc/free memory, so the next loop, the total of 'freed' > always bigger than 10. I am still not sure I follow. Do you mean that there is somebody constantly generating more objects to reclaim? Maybe we are just not agreeing on the definition of an infinite loop but in my book that means that the final condition can never be met. While a busy adding new object might indeed cause drop caches to loop for a long time this is to be expected from that interface as it is supposed to drop all the cache and that can grow during the operation.
On Mon, Sep 14, 2020 at 9:47 PM Michal Hocko <mhocko@suse.com> wrote: > On Mon 14-09-20 21:25:59, Chunxin Zang wrote: > > On Mon, Sep 14, 2020 at 5:30 PM Michal Hocko <mhocko@suse.com> wrote: > > > > > The subject is misleading because this patch doesn't fix an infinite > > > loop, right? It just allows the userspace to interrupt the operation. > > > > > > > > Yes, so we are making a separate patch follow Vlastimil's > recommendations. > > Use double of threshold to end the loop. > > That still means the changelog needs an update > The patch is already merged in Linux-next branch. Can I update the changelog now? This is my first patch, please forgive me :) > > > On Thu, Sep 10, 2020 at 1:59 AM Vlastimil Babka <vbabka@suse.cz> wrote: > > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > > > ... > > > - IMHO it's still worth to bail out in your scenario even without a > > > signal, e.g. > > > by the doubling of threshold. But it can be a separate patch. > > > Thanks! > > > ... > > > > > > > > On Wed 09-09-20 23:20:47, zangchunxin@bytedance.com wrote: > > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > > > > On our server, there are about 10k memcg in one machine. They use > memory > > > > very frequently. When I tigger drop caches，the process will infinite > loop > > > > in drop_slab_node. > > > > > > Is this really an infinite loop, or it just takes a lot of time to > > > process all the metadata in that setup? If this is really an infinite > > > loop then we should look at it. My current understanding is that the > > > operation would finish at some time it just takes painfully long to get > > > there. > > > > > > > Yes, it's really an infinite loop. Every loop spends a lot of time. In > > this time, > > memcg will alloc/free memory, so the next loop, the total of 'freed' > > always bigger than 10. > > I am still not sure I follow. Do you mean that there is somebody > constantly generating more objects to reclaim? > Yes, this is my meaning. :) > Maybe we are just not agreeing on the definition of an infinite loop but > in my book that means that the final condition can never be met. While a > busy adding new object might indeed cause drop caches to loop for a long > time this is to be expected from that interface as it is supposed to > drop all the cache and that can grow during the operation. > -- > Because I have 10k memcg , all of them are heavy users of memory. During each loop, there are always more than 10 reclaimable objects generating, so the condition is never met. The drop cache process has no chance to exit the loop. Although the purpose of the 'drop cache' interface is to release all caches, we still need a way to terminate it, e.g. in this case, the process took too long to run . root 357956 ... R Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches
On Mon 14-09-20 23:02:15, Chunxin Zang wrote: > On Mon, Sep 14, 2020 at 9:47 PM Michal Hocko <mhocko@suse.com> wrote: > > > On Mon 14-09-20 21:25:59, Chunxin Zang wrote: > > > On Mon, Sep 14, 2020 at 5:30 PM Michal Hocko <mhocko@suse.com> wrote: > > > > > > > The subject is misleading because this patch doesn't fix an infinite > > > > loop, right? It just allows the userspace to interrupt the operation. > > > > > > > > > > > Yes, so we are making a separate patch follow Vlastimil's > > recommendations. > > > Use double of threshold to end the loop. > > > > That still means the changelog needs an update > > > > The patch is already merged in Linux-next branch. Can I update the > changelog now? Yes. Andrew will refresh it. He doesn't have a git tree which would prevent rewriting the patch. > This is my first patch, please forgive me :) No worries. The mm patch workflow is rather different from others. > > > On Thu, Sep 10, 2020 at 1:59 AM Vlastimil Babka <vbabka@suse.cz> wrote: > > > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > > > > > ... > > > > - IMHO it's still worth to bail out in your scenario even without a > > > > signal, e.g. > > > > by the doubling of threshold. But it can be a separate patch. > > > > Thanks! > > > > ... > > > > > > > > > > > > On Wed 09-09-20 23:20:47, zangchunxin@bytedance.com wrote: > > > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > > > > > > On our server, there are about 10k memcg in one machine. They use > > memory > > > > > very frequently. When I tigger drop caches，the process will infinite > > loop > > > > > in drop_slab_node. > > > > > > > > Is this really an infinite loop, or it just takes a lot of time to > > > > process all the metadata in that setup? If this is really an infinite > > > > loop then we should look at it. My current understanding is that the > > > > operation would finish at some time it just takes painfully long to get > > > > there. > > > > > > > > > > Yes, it's really an infinite loop. Every loop spends a lot of time. In > > > this time, > > > memcg will alloc/free memory, so the next loop, the total of 'freed' > > > always bigger than 10. > > > > I am still not sure I follow. Do you mean that there is somebody > > constantly generating more objects to reclaim? > > > > Yes, this is my meaning. :) > > > > Maybe we are just not agreeing on the definition of an infinite loop but > > in my book that means that the final condition can never be met. While a > > busy adding new object might indeed cause drop caches to loop for a long > > time this is to be expected from that interface as it is supposed to > > drop all the cache and that can grow during the operation. > > -- > > > > Because I have 10k memcg , all of them are heavy users of memory. > During each loop, there are always more than 10 reclaimable objects > generating, so the > condition is never met. 10k or any number of memcgs shouldn't really make much of a difference. Except for the time the scan adds. Fundamentally we are talking about freed objects and whether they are on the global or per memcg lists should result in a similar behavior. > The drop cache process has no chance to exit the > loop. > Although the purpose of the 'drop cache' interface is to release all > caches, we still need a > way to terminate it, e.g. in this case, the process took too long to run . Yes, this is perfectly understandable. Having a bail out on fatal signal is a completely reasonable thing to do. I am mostly confused by your infinite loop claims and what the relation of this patch to it. I would propose this wording instead " We have observed that drop_caches can take a considerable amount of time (<put data here>). Especially when there are many memcgs involved because they are adding an additional overhead. It is quite unfortunate that the operation cannot be interrupted by a signal currently. Add a check for fatal signals into the main loop so that userspace can control early bailout. " or something along those lines. > > root 357956 ... R Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches
On Mon, Sep 14, 2020 at 11:17 PM Michal Hocko <mhocko@suse.com> wrote: > > On Mon 14-09-20 23:02:15, Chunxin Zang wrote: > > On Mon, Sep 14, 2020 at 9:47 PM Michal Hocko <mhocko@suse.com> wrote: > > > > > On Mon 14-09-20 21:25:59, Chunxin Zang wrote: > > > > On Mon, Sep 14, 2020 at 5:30 PM Michal Hocko <mhocko@suse.com> wrote: > > > > > > > > > The subject is misleading because this patch doesn't fix an infinite > > > > > loop, right? It just allows the userspace to interrupt the operation. > > > > > > > > > > > > > > Yes, so we are making a separate patch follow Vlastimil's > > > recommendations. > > > > Use double of threshold to end the loop. > > > > > > That still means the changelog needs an update > > > > > > > The patch is already merged in Linux-next branch. Can I update the > > changelog now? > > Yes. Andrew will refresh it. He doesn't have a git tree which would > prevent rewriting the patch. > > > This is my first patch, please forgive me :) > > No worries. The mm patch workflow is rather different from others. > > > > > On Thu, Sep 10, 2020 at 1:59 AM Vlastimil Babka <vbabka@suse.cz> wrote: > > > > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > > > > > > > ... > > > > > - IMHO it's still worth to bail out in your scenario even without a > > > > > signal, e.g. > > > > > by the doubling of threshold. But it can be a separate patch. > > > > > Thanks! > > > > > ... > > > > > > > > > > > > > > > > On Wed 09-09-20 23:20:47, zangchunxin@bytedance.com wrote: > > > > > > From: Chunxin Zang <zangchunxin@bytedance.com> > > > > > > > > > > > > On our server, there are about 10k memcg in one machine. They use > > > memory > > > > > > very frequently. When I tigger drop caches，the process will infinite > > > loop > > > > > > in drop_slab_node. > > > > > > > > > > Is this really an infinite loop, or it just takes a lot of time to > > > > > process all the metadata in that setup? If this is really an infinite > > > > > loop then we should look at it. My current understanding is that the > > > > > operation would finish at some time it just takes painfully long to get > > > > > there. > > > > > > > > > > > > > Yes, it's really an infinite loop. Every loop spends a lot of time. In > > > > this time, > > > > memcg will alloc/free memory, so the next loop, the total of 'freed' > > > > always bigger than 10. > > > > > > I am still not sure I follow. Do you mean that there is somebody > > > constantly generating more objects to reclaim? > > > > > > > Yes, this is my meaning. :) > > > > > > > Maybe we are just not agreeing on the definition of an infinite loop but > > > in my book that means that the final condition can never be met. While a > > > busy adding new object might indeed cause drop caches to loop for a long > > > time this is to be expected from that interface as it is supposed to > > > drop all the cache and that can grow during the operation. > > > -- > > > > > > > Because I have 10k memcg , all of them are heavy users of memory. > > During each loop, there are always more than 10 reclaimable objects > > generating, so the > > condition is never met. > > 10k or any number of memcgs shouldn't really make much of a difference. > Except for the time the scan adds. Fundamentally we are talking about > freed objects and whether they are on the global or per memcg lists > should result in a similar behavior. > > > The drop cache process has no chance to exit the > > loop. > > Although the purpose of the 'drop cache' interface is to release all > > caches, we still need a > > way to terminate it, e.g. in this case, the process took too long to run . > > Yes, this is perfectly understandable. Having a bail out on fatal signal > is a completely reasonable thing to do. I am mostly confused by your > infinite loop claims and what the relation of this patch to it. I would > propose this wording instead > > " > We have observed that drop_caches can take a considerable amount of > time (<put data here>). Especially when there are many memcgs involved > because they are adding an additional overhead. > > It is quite unfortunate that the operation cannot be interrupted by a > signal currently. Add a check for fatal signals into the main loop > so that userspace can control early bailout. > " I understand what you mean. Yes, you are right, The patch only provides a way for users to terminate the process, not to reduce time consumption, not improve the situation about loops. I will update the description of the patch in v3. Thanks so much :) > or something along those lines. > > > > > root 357956 ... R Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches > > -- > Michal Hocko > SUSE Labs Best wishes Chunxin
diff --git a/mm/vmscan.c b/mm/vmscan.c index b6d84326bdf2..c3ed8b45d264 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -704,6 +704,9 @@ void drop_slab_node(int nid) do { struct mem_cgroup *memcg = NULL; + if (fatal_signal_pending(current)) + return; + freed = 0; memcg = mem_cgroup_iter(NULL, NULL, NULL); do {