Messages in this thread | | | Date | Fri, 24 Feb 2023 12:00:21 +0800 | Subject | Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless | From | Qi Zheng <> |
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On 2023/2/24 02:24, Sultan Alsawaf wrote: > On Thu, Feb 23, 2023 at 09:27:20PM +0800, Qi Zheng wrote: >> The shrinker_rwsem is a global lock in shrinkers subsystem, >> it is easy to cause blocking in the following cases: >> >> a. the write lock of shrinker_rwsem was held for too long. >> For example, there are many memcgs in the system, which >> causes some paths to hold locks and traverse it for too >> long. (e.g. expand_shrinker_info()) >> b. the read lock of shrinker_rwsem was held for too long, >> and a writer came at this time. Then this writer will be >> forced to wait and block all subsequent readers. >> For example: >> - be scheduled when the read lock of shrinker_rwsem is >> held in do_shrink_slab() >> - some shrinker are blocked for too long. Like the case >> mentioned in the patchset[1]. >> >> Therefore, many times in history ([2],[3],[4],[5]), some >> people wanted to replace shrinker_rwsem reader with SRCU, >> but they all gave up because SRCU was not unconditionally >> enabled. >> >> But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"), >> the SRCU is unconditionally enabled. So it's time to use >> SRCU to protect readers who previously held shrinker_rwsem. >> >> [1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@virtuozzo.com/ >> [2]. https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/ >> [3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/ >> [4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/ >> [5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/ >> >> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com> >> --- >> mm/vmscan.c | 27 +++++++++++---------------- >> 1 file changed, 11 insertions(+), 16 deletions(-) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index 9f895ca6216c..02987a6f95d1 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -202,6 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task, >> >> LIST_HEAD(shrinker_list); >> DECLARE_RWSEM(shrinker_rwsem); >> +DEFINE_SRCU(shrinker_srcu); >> >> #ifdef CONFIG_MEMCG >> static int shrinker_nr_max; >> @@ -706,7 +707,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker) >> void register_shrinker_prepared(struct shrinker *shrinker) >> { >> down_write(&shrinker_rwsem); >> - list_add_tail(&shrinker->list, &shrinker_list); >> + list_add_tail_rcu(&shrinker->list, &shrinker_list); >> shrinker->flags |= SHRINKER_REGISTERED; >> shrinker_debugfs_add(shrinker); >> up_write(&shrinker_rwsem); >> @@ -760,13 +761,15 @@ void unregister_shrinker(struct shrinker *shrinker) >> return; >> >> down_write(&shrinker_rwsem); >> - list_del(&shrinker->list); >> + list_del_rcu(&shrinker->list); >> shrinker->flags &= ~SHRINKER_REGISTERED; >> if (shrinker->flags & SHRINKER_MEMCG_AWARE) >> unregister_memcg_shrinker(shrinker); >> debugfs_entry = shrinker_debugfs_remove(shrinker); >> up_write(&shrinker_rwsem); >> >> + synchronize_srcu(&shrinker_srcu); >> + >> debugfs_remove_recursive(debugfs_entry); >> >> kfree(shrinker->nr_deferred); >> @@ -786,6 +789,7 @@ void synchronize_shrinkers(void) >> { >> down_write(&shrinker_rwsem); >> up_write(&shrinker_rwsem); >> + synchronize_srcu(&shrinker_srcu); >> } >> EXPORT_SYMBOL(synchronize_shrinkers); >> >> @@ -996,6 +1000,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >> { >> unsigned long ret, freed = 0; >> struct shrinker *shrinker; >> + int srcu_idx; >> >> /* >> * The root memcg might be allocated even though memcg is disabled >> @@ -1007,10 +1012,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >> if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg)) >> return shrink_slab_memcg(gfp_mask, nid, memcg, priority); >> >> - if (!down_read_trylock(&shrinker_rwsem)) >> - goto out; >> + srcu_idx = srcu_read_lock(&shrinker_srcu); >> >> - list_for_each_entry(shrinker, &shrinker_list, list) { >> + list_for_each_entry_srcu(shrinker, &shrinker_list, list, >> + srcu_read_lock_held(&shrinker_srcu)) { >> struct shrink_control sc = { >> .gfp_mask = gfp_mask, >> .nid = nid, >> @@ -1021,19 +1026,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >> if (ret == SHRINK_EMPTY) >> ret = 0; >> freed += ret; >> - /* >> - * Bail out if someone want to register a new shrinker to >> - * prevent the registration from being stalled for long periods >> - * by parallel ongoing shrinking. >> - */ >> - if (rwsem_is_contended(&shrinker_rwsem)) { >> - freed = freed ? : 1; >> - break; >> - } >> } >> >> - up_read(&shrinker_rwsem); >> -out: >> + srcu_read_unlock(&shrinker_srcu, srcu_idx); >> cond_resched(); >> return freed; >> } >> -- >> 2.20.1 >> >> > > Hi Qi, > > A different problem I realized after my old attempt to use SRCU was that the > unregister_shrinker() path became quite slow due to the heavy synchronize_srcu() > call. Both register_shrinker() *and* unregister_shrinker() are called frequently > these days, and SRCU is too unfair to the unregister path IMO.
Hi Sultan,
IIUC, for unregister_shrinker(), the wait time is hardly longer with SRCU than with shrinker_rwsem before.
And I just did a simple test. After using the script in cover letter to increase the shrink_slab hotspot, I did umount 1k times at the same time, and then I used bpftrace to measure the time consumption of unregister_shrinker() as follows:
bpftrace -e 'kprobe:unregister_shrinker { @start[tid] = nsecs; } kretprobe:unregister_shrinker /@start[tid]/ { @ns[comm] = hist(nsecs - @start[tid]); delete(@start[tid]); }'
@ns[umount]: [16K, 32K) 3 | | [32K, 64K) 66 |@@@@@@@@@@ | [64K, 128K) 32 |@@@@@ | [128K, 256K) 22 |@@@ | [256K, 512K) 48 |@@@@@@@ | [512K, 1M) 19 |@@@ | [1M, 2M) 131 |@@@@@@@@@@@@@@@@@@@@@ | [2M, 4M) 313 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [4M, 8M) 302 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ | [8M, 16M) 55 |@@@@@@@@@
I see that the highest time-consuming of unregister_shrinker() is between 8ms and 16ms, which feels tolerable?
Thanks, Qi
> > Although I never got around to submitting it, I made a non-SRCU solution [1] > that uses fine-grained locking instead, which is fair to both the register path > and unregister path. (The patch I've linked is a version of this adapted to an > older 4.14 kernel FYI, but it can be reworked for the current kernel.) > > What do you think about the fine-grained locking approach? > > Thanks, > Sultan > > [1] https://github.com/kerneltoast/android_kernel_google_floral/commit/012378f3173a82d2333d3ae7326691544301e76a >
-- Thanks, Qi
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