| Message ID | 20220309011824.1454619-1-longman@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [PATCH-mm,v2] mm/list_lru: Optimize memcg_reparent_list_lru_node() | expand |
On Tue, Mar 08, 2022 at 08:18:24PM -0500, Waiman Long wrote: > Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() > to be race free"), we are tracking the total number of lru > entries in a list_lru_node in its nr_items field. In the case of > memcg_reparent_list_lru_node(), there is nothing to be done if nr_items > is 0. We don't even need to take the nlru->lock as no new lru entry > could be added by a racing list_lru_add() to the draining src_idx memcg > at this point. > > Signed-off-by: Waiman Long <longman@redhat.com> > --- > mm/list_lru.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/mm/list_lru.c b/mm/list_lru.c > index ba76428ceece..c669d87001a6 100644 > --- a/mm/list_lru.c > +++ b/mm/list_lru.c > @@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid, > int dst_idx = dst_memcg->kmemcg_id; > struct list_lru_one *src, *dst; > > + /* > + * If there is no lru entry in this nlru, we can skip it immediately. > + */ > + if (!READ_ONCE(nlru->nr_items)) > + return; This is a per-node counter, not a per-memcg, right? If so, do we optimize for the case when all lru items belong to one node and others are empty?
On 3/8/22 21:13, Roman Gushchin wrote: > On Tue, Mar 08, 2022 at 08:18:24PM -0500, Waiman Long wrote: >> Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() >> to be race free"), we are tracking the total number of lru >> entries in a list_lru_node in its nr_items field. In the case of >> memcg_reparent_list_lru_node(), there is nothing to be done if nr_items >> is 0. We don't even need to take the nlru->lock as no new lru entry >> could be added by a racing list_lru_add() to the draining src_idx memcg >> at this point. >> >> Signed-off-by: Waiman Long <longman@redhat.com> >> --- >> mm/list_lru.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >> >> diff --git a/mm/list_lru.c b/mm/list_lru.c >> index ba76428ceece..c669d87001a6 100644 >> --- a/mm/list_lru.c >> +++ b/mm/list_lru.c >> @@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid, >> int dst_idx = dst_memcg->kmemcg_id; >> struct list_lru_one *src, *dst; >> >> + /* >> + * If there is no lru entry in this nlru, we can skip it immediately. >> + */ >> + if (!READ_ONCE(nlru->nr_items)) >> + return; > This is a per-node counter, not a per-memcg, right? Right. list_lru_node is a per-node structure inside list_lru. > If so, do we optimize for the case when all lru items belong to one node and > others are empty? That is actually the case that I am trying to optimize for. If a system has many containers. It is also likely each container may mount one or more container specific filesystems. Since a container likely use just a few cpus, it is highly that only the list_lru_node that contains those cpus will be utilized while the rests may be empty. I got the idea of doing this patch when I was looking at a crash dump related to the list_lru code. That particular crash dump has more than 13k list_lru's and thousands of mount points. I had notice even if nr_items of a list_lru_node is 0, it still tries to transfer lru entries from source idx to dest idx. Without doing an lock/unlock and loading a cacheline from the memcg_lrus, it can save some time. That can be substantial saving if we are talking about thousands of list_lru's. Cheers, Longman
On Tue, Mar 08, 2022 at 10:12:48PM -0500, Waiman Long wrote: > On 3/8/22 21:13, Roman Gushchin wrote: > > On Tue, Mar 08, 2022 at 08:18:24PM -0500, Waiman Long wrote: > > > Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() > > > to be race free"), we are tracking the total number of lru > > > entries in a list_lru_node in its nr_items field. In the case of > > > memcg_reparent_list_lru_node(), there is nothing to be done if nr_items > > > is 0. We don't even need to take the nlru->lock as no new lru entry > > > could be added by a racing list_lru_add() to the draining src_idx memcg > > > at this point. > > > > > > Signed-off-by: Waiman Long <longman@redhat.com> > > > --- > > > mm/list_lru.c | 6 ++++++ > > > 1 file changed, 6 insertions(+) > > > > > > diff --git a/mm/list_lru.c b/mm/list_lru.c > > > index ba76428ceece..c669d87001a6 100644 > > > --- a/mm/list_lru.c > > > +++ b/mm/list_lru.c > > > @@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid, > > > int dst_idx = dst_memcg->kmemcg_id; > > > struct list_lru_one *src, *dst; > > > + /* > > > + * If there is no lru entry in this nlru, we can skip it immediately. > > > + */ > > > + if (!READ_ONCE(nlru->nr_items)) > > > + return; > > This is a per-node counter, not a per-memcg, right? > Right. list_lru_node is a per-node structure inside list_lru. > > If so, do we optimize for the case when all lru items belong to one node and > > others are empty? > > That is actually the case that I am trying to optimize for. > > If a system has many containers. It is also likely each container may mount > one or more container specific filesystems. Since a container likely use > just a few cpus, it is highly that only the list_lru_node that contains > those cpus will be utilized while the rests may be empty. > > I got the idea of doing this patch when I was looking at a crash dump > related to the list_lru code. That particular crash dump has more than 13k > list_lru's and thousands of mount points. I had notice even if nr_items of a > list_lru_node is 0, it still tries to transfer lru entries from source idx > to dest idx. Without doing an lock/unlock and loading a cacheline from the > memcg_lrus, it can save some time. That can be substantial saving if we are > talking about thousands of list_lru's. Cool! Makes total sense to me. Thanks for the explanation! Would you mind to add this text to the commit log? Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev> Thanks!
On 3/8/22 23:46, Roman Gushchin wrote: > On Tue, Mar 08, 2022 at 10:12:48PM -0500, Waiman Long wrote: >> On 3/8/22 21:13, Roman Gushchin wrote: >>> On Tue, Mar 08, 2022 at 08:18:24PM -0500, Waiman Long wrote: >>>> Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() >>>> to be race free"), we are tracking the total number of lru >>>> entries in a list_lru_node in its nr_items field. In the case of >>>> memcg_reparent_list_lru_node(), there is nothing to be done if nr_items >>>> is 0. We don't even need to take the nlru->lock as no new lru entry >>>> could be added by a racing list_lru_add() to the draining src_idx memcg >>>> at this point. >>>> >>>> Signed-off-by: Waiman Long <longman@redhat.com> >>>> --- >>>> mm/list_lru.c | 6 ++++++ >>>> 1 file changed, 6 insertions(+) >>>> >>>> diff --git a/mm/list_lru.c b/mm/list_lru.c >>>> index ba76428ceece..c669d87001a6 100644 >>>> --- a/mm/list_lru.c >>>> +++ b/mm/list_lru.c >>>> @@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid, >>>> int dst_idx = dst_memcg->kmemcg_id; >>>> struct list_lru_one *src, *dst; >>>> + /* >>>> + * If there is no lru entry in this nlru, we can skip it immediately. >>>> + */ >>>> + if (!READ_ONCE(nlru->nr_items)) >>>> + return; >>> This is a per-node counter, not a per-memcg, right? >> Right. list_lru_node is a per-node structure inside list_lru. >>> If so, do we optimize for the case when all lru items belong to one node and >>> others are empty? >> That is actually the case that I am trying to optimize for. >> >> If a system has many containers. It is also likely each container may mount >> one or more container specific filesystems. Since a container likely use >> just a few cpus, it is highly that only the list_lru_node that contains >> those cpus will be utilized while the rests may be empty. >> >> I got the idea of doing this patch when I was looking at a crash dump >> related to the list_lru code. That particular crash dump has more than 13k >> list_lru's and thousands of mount points. I had notice even if nr_items of a >> list_lru_node is 0, it still tries to transfer lru entries from source idx >> to dest idx. Without doing an lock/unlock and loading a cacheline from the >> memcg_lrus, it can save some time. That can be substantial saving if we are >> talking about thousands of list_lru's. > Cool! Makes total sense to me. Thanks for the explanation! > Would you mind to add this text to the commit log? > > Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev> Sure. I will add some of it into the commit log. Cheers, Longman
diff --git a/mm/list_lru.c b/mm/list_lru.c index ba76428ceece..c669d87001a6 100644 --- a/mm/list_lru.c +++ b/mm/list_lru.c @@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid, int dst_idx = dst_memcg->kmemcg_id; struct list_lru_one *src, *dst; + /* + * If there is no lru entry in this nlru, we can skip it immediately. + */ + if (!READ_ONCE(nlru->nr_items)) + return; + /* * Since list_lru_{add,del} may be called under an IRQ-safe lock, * we have to use IRQ-safe primitives here to avoid deadlock.
Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() to be race free"), we are tracking the total number of lru entries in a list_lru_node in its nr_items field. In the case of memcg_reparent_list_lru_node(), there is nothing to be done if nr_items is 0. We don't even need to take the nlru->lock as no new lru entry could be added by a racing list_lru_add() to the draining src_idx memcg at this point. Signed-off-by: Waiman Long <longman@redhat.com> --- mm/list_lru.c | 6 ++++++ 1 file changed, 6 insertions(+)