| Message ID | d1fc50d1-8a64-a118-7040-8ae5606d411d@virtuozzo.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 12.07.2018 14:13, Kirill Tkhai wrote: > On 03.07.2018 20:32, Kirill Tkhai wrote: >> On 03.07.2018 20:00, Shakeel Butt wrote: >>> On Tue, Jul 3, 2018 at 9:17 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote: >>>> >>>> Hi, Shakeel, >>>> >>>> On 03.07.2018 18:46, Shakeel Butt wrote: >>>>> On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox <willy@infradead.org> wrote: >>>>>> >>>>>> On Tue, Jul 03, 2018 at 06:09:05PM +0300, Kirill Tkhai wrote: >>>>>>> +++ b/mm/vmscan.c >>>>>>> @@ -169,6 +169,49 @@ unsigned long vm_total_pages; >>>>>>> static LIST_HEAD(shrinker_list); >>>>>>> static DECLARE_RWSEM(shrinker_rwsem); >>>>>>> >>>>>>> +#ifdef CONFIG_MEMCG_KMEM >>>>>>> +static DEFINE_IDR(shrinker_idr); >>>>>>> +static int shrinker_nr_max; >>>>>> >>>>>> So ... we've now got a list_head (shrinker_list) which contains all of >>>>>> the shrinkers, plus a shrinker_idr which contains the memcg-aware shrinkers? >>>>>> >>>>>> Why not replace the shrinker_list with the shrinker_idr? It's only used >>>>>> twice in vmscan.c: >>>>>> >>>>>> void register_shrinker_prepared(struct shrinker *shrinker) >>>>>> { >>>>>> down_write(&shrinker_rwsem); >>>>>> list_add_tail(&shrinker->list, &shrinker_list); >>>>>> up_write(&shrinker_rwsem); >>>>>> } >>>>>> >>>>>> list_for_each_entry(shrinker, &shrinker_list, list) { >>>>>> ... >>>>>> >>>>>> The first is simply idr_alloc() and the second is >>>>>> >>>>>> idr_for_each_entry(&shrinker_idr, shrinker, id) { >>>>>> >>>>>> I understand there's a difference between allocating the shrinker's ID and >>>>>> adding it to the list. You can do this by calling idr_alloc with NULL >>>>>> as the pointer, and then using idr_replace() when you want to add the >>>>>> shrinker to the list. idr_for_each_entry() skips over NULL entries. >>>>>> >>>>>> This will actually reduce the size of each shrinker and be more >>>>>> cache-efficient when calling the shrinkers. I think we can also get >>>>>> rid of the shrinker_rwsem eventually, but let's leave it for now. >>>>> >>>>> Can you explain how you envision shrinker_rwsem can be removed? I am >>>>> very much interested in doing that. >>>> >>>> Have you tried to do some games with SRCU? It looks like we just need to >>>> teach count_objects() and scan_objects() to work with semi-destructed >>>> shrinkers. Though, this looks this will make impossible to introduce >>>> shrinkers, which do synchronize_srcu() in scan_objects() for example. >>>> Not sure, someone will actually use this, and this is possible to consider >>>> as limitation. >>>> >>> >>> Hi Kirill, I tried SRCU and the discussion is at >>> https://lore.kernel.org/lkml/20171117173521.GA21692@infradead.org/T/#u >>> >>> Paul E. McKenney suggested to enable SRCU unconditionally. So, to use >>> SRCU for shrinkers, we first have to push unconditional SRCU. >> >> First time, I read this, I though the talk goes about some new srcu_read_lock() >> without an argument and it's need to rework SRCU in some huge way. Thanks >> god, it was just a misreading :) >>> Tetsuo had another lockless solution which was a bit involved but does >>> not depend on SRCU. >> >> Ok, I see refcounters suggestion. Thanks for the link, Shakeel! > > Just returning to this theme. Since both of the suggested ways contain > srcu synchronization, it may be better just to use percpu-rwsem, since > there is the same functionality out-of-box. > > register/unregister_shrinker() will use two rw semaphores: > > register_shrinker() > { > down_write(&shrinker_rwsem); > idr_alloc(); > up_write(&shrinker_rwsem); > } > > unregister_shrinker() > { > percpu_down_write(&percpu_shrinker_rwsem); > down_write(&shrinker_rwsem); > idr_remove(); > up_write(&shrinker_rwsem); > percpu_up_write(&percpu_shrinker_rwsem); > } > > shrink_slab() > { > percpu_down_read(&percpu_shrinker_rwsem); > rcu_read_lock(); > shrinker = idr_find(); > rcu_read_unlock(); > > do_shrink_slab(shrinker); > percpu_up_read(&percpu_shrinker_rwsem); > } > > 1)Here is a trick to make register_shrinker() not use percpu semaphore, > i.e., not to wait RCU synchronization. This just makes register_shrinker() > faster. So, we introduce 2 semaphores instead of 1: > shrinker_rwsem to protect IDR and percpu_shrinker_rwsem. > > 2)rcu_read_lock() -- to synchronize idr_find() with idr_alloc(). > Not sure, we really need this. It's possible, lockless idr_find() > is OK in parallel with allocation of new ID. Parallel removing > is not possible because of percpu rwsem. > > 3)Places, which are performance critical to unregister_shrinker() speed > (e.g., like deactivate_locked_super(), as we want umount() to be fast), > may just call it delayed from work: > > diff --git a/fs/super.c b/fs/super.c > index 13647d4fd262..b4a98cb00166 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -324,19 +324,7 @@ void deactivate_locked_super(struct super_block *s) > struct file_system_type *fs = s->s_type; > if (atomic_dec_and_test(&s->s_active)) { > cleancache_invalidate_fs(s); > - unregister_shrinker(&s->s_shrink); > - fs->kill_sb(s); > - > - /* > - * Since list_lru_destroy() may sleep, we cannot call it from > - * put_super(), where we hold the sb_lock. Therefore we destroy > - * the lru lists right now. > - */ > - list_lru_destroy(&s->s_dentry_lru); > - list_lru_destroy(&s->s_inode_lru); > - > - put_filesystem(fs); > - put_super(s); > + schedule_delayed_deactivate_super(s) > } else { > up_write(&s->s_umount); > } s/shrinker_rwsem/shrinker_mutex/
diff --git a/fs/super.c b/fs/super.c index 13647d4fd262..b4a98cb00166 100644 --- a/fs/super.c +++ b/fs/super.c @@ -324,19 +324,7 @@ void deactivate_locked_super(struct super_block *s) struct file_system_type *fs = s->s_type; if (atomic_dec_and_test(&s->s_active)) { cleancache_invalidate_fs(s); - unregister_shrinker(&s->s_shrink); - fs->kill_sb(s); - - /* - * Since list_lru_destroy() may sleep, we cannot call it from - * put_super(), where we hold the sb_lock. Therefore we destroy - * the lru lists right now. - */ - list_lru_destroy(&s->s_dentry_lru); - list_lru_destroy(&s->s_inode_lru); - - put_filesystem(fs); - put_super(s); + schedule_delayed_deactivate_super(s) } else { up_write(&s->s_umount); }