Messages in this thread | | | Date | Thu, 1 Jun 2023 18:44:37 +0800 | Subject | Re: [linus:master] [mm] f95bdb700b: stress-ng.ramfs.ops_per_sec -88.8% regression | From | Qi Zheng <> |
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Hi Kirill,
On 2023/6/1 18:06, Kirill Tkhai wrote: > 01.06.2023, 11:34, "Qi Zheng" <qi.zheng@linux.dev > <mailto:qi.zheng@linux.dev>>: > > > > On 2023/6/1 08:57, Kirill Tkhai wrote: > > Hi, > > On 30.05.2023 06:07, Qi Zheng wrote: > > Hi, > > On 2023/5/29 20:51, Paul E. McKenney wrote: > > On Mon, May 29, 2023 at 10:39:21AM +0800, Qi Zheng wrote: > > > [...] > > > Thanks for such a detailed explanation. > > Now I think we can continue to try to complete the > idea[1] from > Kirill Tkhai. The patch moves heavy > synchronize_srcu() to delayed > work, so it doesn't affect on user-visible > unregistration speed. > > [1]. > https://lore.kernel.org/lkml/153365636747.19074.12610817307548583381.stgit@localhost.localdomain/ <https://lore.kernel.org/lkml/153365636747.19074.12610817307548583381.stgit@localhost.localdomain/> > > > A blast from the past! ;-) > > But yes, moving the long-latency synchronize_srcu() > off the user-visible > critical code path can be even better. > > > Yeah, I applied these patches ([PATCH RFC 04/10]~[PATCH > RFC 10/10], > with few conflicts), the ops/s does get back to the > previous levels. > > I'll continue updating this patchset after doing more testing. > > > You may also fix the issue using the below generic solution. > > In addition to this we need patch, which calls > unregister_shrinker_delayed_initiate() > instead of unregister_shrinker() in deactivate_locked_super(), > and calls > unregister_shrinker_delayed_finalize() from > destroy_super_work(). Compilation tested only. > > --- > include/linux/shrinker.h | 2 ++ > mm/vmscan.c | 38 ++++++++++++++++++++++++++++++-------- > 2 files changed, 32 insertions(+), 8 deletions(-) > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h > index 224293b2dd06..4ba2986716d3 100644 > --- a/include/linux/shrinker.h > +++ b/include/linux/shrinker.h > @@ -4,6 +4,7 @@ > > #include <linux/atomic.h> > #include <linux/types.h> > +#include <linux/rwsem.h> > > /* > * This struct is used to pass information from page reclaim > to the shrinkers. > @@ -83,6 +84,7 @@ struct shrinker { > #endif > /* objs pending delete, per node */ > atomic_long_t *nr_deferred; > + struct rw_semaphore rwsem; > }; > #define DEFAULT_SEEKS 2 /* A good number if you don't know > better. */ > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index eeca83e28c9b..19fc129771ce 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -706,6 +706,7 @@ static int __prealloc_shrinker(struct > shrinker *shrinker) > if (!shrinker->nr_deferred) > return -ENOMEM; > > + init_rwsem(&shrinker->rwsem); > return 0; > } > > @@ -757,7 +758,9 @@ void register_shrinker_prepared(struct > shrinker *shrinker) > { > mutex_lock(&shrinker_mutex); > list_add_tail_rcu(&shrinker->list, &shrinker_list); > + down_write(&shrinker->rwsem); > shrinker->flags |= SHRINKER_REGISTERED; > + up_write(&shrinker->rwsem); > shrinker_debugfs_add(shrinker); > mutex_unlock(&shrinker_mutex); > } > @@ -802,7 +805,7 @@ EXPORT_SYMBOL(register_shrinker); > /* > * Remove one > */ > -void unregister_shrinker(struct shrinker *shrinker) > +void unregister_shrinker_delayed_initiate(struct shrinker > *shrinker) > { > struct dentry *debugfs_entry; > int debugfs_id; > @@ -812,20 +815,33 @@ void unregister_shrinker(struct shrinker > *shrinker) > > mutex_lock(&shrinker_mutex); > list_del_rcu(&shrinker->list); > + down_write(&shrinker->rwsem); > shrinker->flags &= ~SHRINKER_REGISTERED; > + up_write(&shrinker->rwsem); > if (shrinker->flags & SHRINKER_MEMCG_AWARE) > unregister_memcg_shrinker(shrinker); > debugfs_entry = shrinker_debugfs_detach(shrinker, > &debugfs_id); > mutex_unlock(&shrinker_mutex); > > + shrinker_debugfs_remove(debugfs_entry, debugfs_id); // This > is moved in your patch > +} > +EXPORT_SYMBOL(unregister_shrinker_delayed_initiate); > + > +void unregister_shrinker_delayed_finalize(struct shrinker > *shrinker) > +{ > atomic_inc(&shrinker_srcu_generation); > synchronize_srcu(&shrinker_srcu); > > - shrinker_debugfs_remove(debugfs_entry, debugfs_id); > - > kfree(shrinker->nr_deferred); > shrinker->nr_deferred = NULL; > } > +EXPORT_SYMBOL(unregister_shrinker_delayed_finalize); > + > +void unregister_shrinker(struct shrinker *shrinker) > +{ > + unregister_shrinker_delayed_initiate(shrinker); > + unregister_shrinker_delayed_finalize(shrinker); > +} > EXPORT_SYMBOL(unregister_shrinker); > > /** > @@ -856,9 +872,14 @@ static unsigned long > do_shrink_slab(struct shrink_control *shrinkctl, > : SHRINK_BATCH; > long scanned = 0, next_deferred; > > + down_read(&shrinker->rwsem); > + if (!(shrinker->flags & SHRINKER_REGISTERED)) > + goto unlock; > freeable = shrinker->count_objects(shrinker, shrinkctl); > - if (freeable == 0 || freeable == SHRINK_EMPTY) > - return freeable; > + if (freeable == 0 || freeable == SHRINK_EMPTY) { > + freed = freeable; > + goto unlock; > + } > > /* > * copy the current shrinker scan count into a local > variable > @@ -935,6 +956,8 @@ static unsigned long do_shrink_slab(struct > shrink_control *shrinkctl, > * manner that handles concurrent updates. > */ > new_nr = add_nr_deferred(next_deferred, shrinker, > shrinkctl); > +unlock: > + up_read(&shrinker->rwsem); > > > It should be moved after trace_mm_shrink_slab_end(). > > Could you explain the reason? I don't see the variable it will protect.
We jump to unlock label before trace_mm_shrink_slab_start(), so I think we should not go to call trace_mm_shrink_slab_end().
> > > trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, > freed, nr, new_nr, total_scan); > return freed; > @@ -968,9 +991,8 @@ static unsigned long > shrink_slab_memcg(gfp_t gfp_mask, int nid, > struct shrinker *shrinker; > > shrinker = idr_find(&shrinker_idr, i); > - if (unlikely(!shrinker || !(shrinker->flags & > SHRINKER_REGISTERED))) { > - if (!shrinker) > - clear_bit(i, info->map); > + if (unlikely(!shrinker)) { > + clear_bit(i, info->map); > continue; > } > > > Keep this as a fast path? > > Probably, yes. And also we need 1)down_trylock() instead of down_read() > and 2)rwsem_is_contended in do_shrink_slab().
Agree, although the critical section of the writer of shrinker->rwsem is very short, this prevents unnecessary sleeps.
> > > After applying the above patch, the performance regression problem of > ops/s can be solved. And it can be guaranteed that the shrinker is not > running after unregister_shrinker_delayed_initiate(), so the previous > semantics are not broken. > > Keeping old semantics or not is quite subjective, I think. It's possible > to provide strong arguments for both cases. First is faster, second is > easier to adopt for users. For me personally the faster approach looks > better.
Agree. I also like completely lock-less slab shrink.
> >nce the lock granularity of down_read() has changed to the granularity > > of per shrinker, it seems that the down_read() perf hotspot will not be > very high. I'm not quite sure why. > > (The test script used is the script in > https://lore.kernel.org/all/20230313112819.38938-4-zhengqi.arch@bytedance.com/ <https://lore.kernel.org/all/20230313112819.38938-4-zhengqi.arch@bytedance.com/>) > > Hmm, possible original shrinker_rwsem had a lot of atomic intersections > between cpus on down_read(), while with small locks it has not. Are
I guess so.
> CONFIG_ for locks debug are same in original and this case?
Basically yes, I will do some more testing.
Thanks, Qi
> > > 25.28% [kernel] [k] do_shrink_slab > 21.91% [kernel] [k] pv_native_safe_halt > 10.81% [kernel] [k] _find_next_bit > 10.47% [kernel] [k] down_read > 8.75% [kernel] [k] shrink_slab > 4.03% [kernel] [k] up_read > 3.29% [kernel] [k] shrink_lruvec > 2.75% [kernel] [k] xa_load > 2.73% [kernel] [k] mem_cgroup_iter > 2.67% [kernel] [k] shrink_node > 1.30% [kernel] [k] list_lru_count_one > > Thanks, > Qi > >
-- Thanks, Qi
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