On Tue, Jul 07, 2020 at 01:06:51AM +0000, Dennis Zhou wrote: > On Mon, Jul 06, 2020 at 09:24:43PM +0800, Feng Tang wrote: > > Hi All, > > > > Please help to review this fix patch, thanks! > > > > It is against today's linux-mm tree. For easy review, I put the fix > > into one patch, and I could split it to 2 parts for percpu-counter > > and mm/util.c if it's preferred. > > > > From 593f9dc139181a7c3bb1705aacd1f625f400e458 Mon Sep 17 00:00:00 2001 > > From: Feng Tang <feng.tang@xxxxxxxxx> > > Date: Mon, 6 Jul 2020 14:48:29 +0800 > > Subject: [PATCH] mm/util.c: sync vm_committed_as when changing memory policy > > to OVERCOMMIT_NEVER > > > > With the patch to improve scalability of vm_committed_as [1], 0day reported > > the ltp overcommit_memory test case could fail (fail rate is about 5/50) [2]. > > The root cause is when system is running with loose memory overcommit policy > > like OVERCOMMIT_GUESS/ALWAYS, the deviation of vm_committed_as could be big, > > and once the policy is runtime changed to OVERCOMMIT_NEVER, vm_committed_as's > > batch is decreased to 1/64 of original one, but the deviation is not > > compensated accordingly, and following __vm_enough_memory() check for vm > > overcommit could be wrong due to this deviation, which breaks the ltp > > overcommit_memory case. > > > > Fix it by forcing a sync for percpu counter vm_committed_as when overcommit > > policy is changed to OVERCOMMIT_NEVER (sysctl -w vm.overcommit_memory=2). > > The sync itself is not a fast operation, and is toleratable given user is > > not expected to frequently changing policy to OVERCOMMIT_NEVER. > > > > [1] https://lore.kernel.org/lkml/1592725000-73486-1-git-send-email-feng.tang@xxxxxxxxx/ > > [2] https://marc.info/?l=linux-mm&m=159367156428286 (can't find a link in lore.kernel.org) > > > > Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx> > > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx> > > --- > > include/linux/percpu_counter.h | 4 ++++ > > lib/percpu_counter.c | 14 ++++++++++++++ > > mm/util.c | 11 ++++++++++- > > 3 files changed, 28 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h > > index 0a4f54d..01861ee 100644 > > --- a/include/linux/percpu_counter.h > > +++ b/include/linux/percpu_counter.h > > @@ -44,6 +44,7 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, > > s32 batch); > > s64 __percpu_counter_sum(struct percpu_counter *fbc); > > int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch); > > +void percpu_counter_sync(struct percpu_counter *fbc); > > > > static inline int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs) > > { > > @@ -172,6 +173,9 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc) > > return true; > > } > > > > +static inline void percpu_counter_sync(struct percpu_counter *fbc) > > +{ > > +} > > #endif /* CONFIG_SMP */ > > > > static inline void percpu_counter_inc(struct percpu_counter *fbc) > > diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c > > index a66595b..02d87fc 100644 > > --- a/lib/percpu_counter.c > > +++ b/lib/percpu_counter.c > > @@ -98,6 +98,20 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch) > > } > > EXPORT_SYMBOL(percpu_counter_add_batch); > > > > +void percpu_counter_sync(struct percpu_counter *fbc) > > +{ > > + unsigned long flags; > > + s64 count; > > + > > + raw_spin_lock_irqsave(&fbc->lock, flags); > > + count = __this_cpu_read(*fbc->counters); > > + fbc->count += count; > > + __this_cpu_sub(*fbc->counters, count); > > + raw_spin_unlock_irqrestore(&fbc->lock, flags); > > +} > > +EXPORT_SYMBOL(percpu_counter_sync); > > + > > + > > /* > > * Add up all the per-cpu counts, return the result. This is a more accurate > > * but much slower version of percpu_counter_read_positive() > > diff --git a/mm/util.c b/mm/util.c > > index 52ed9c1..5fb62c0 100644 > > --- a/mm/util.c > > +++ b/mm/util.c > > @@ -746,14 +746,23 @@ int overcommit_ratio_handler(struct ctl_table *table, int write, void *buffer, > > return ret; > > } > > > > +static void sync_overcommit_as(struct work_struct *dummy) > > +{ > > + percpu_counter_sync(&vm_committed_as); > > +} > > + > > This seems like a rather niche use case as it's currently coupled with a > schedule_on_each_cpu(). I can't imagine a use case where you'd want to > do this without being called by schedule_on_each_cpu(). Yes! > > Would it be better to modify or introduce something akin to > percpu_counter_sum() which sums and folds in the counter state? I'd be > curious to see what the cost of always folding would be as this is > already considered the cold path and would help with the next batch too. Initially, I also thought about doing the sync just like percpu_counter_sum(): raw_spin_lock_irqsave for_each_online_cpu(cpu) } do-the-sync raw_spin_unlock_irqrestore One problem is the per_cpu_ptr(fbc->counters, cpu) could still be updated on other CPUs as the fast path update is not protected by fbc->lock. As for cost, it is about about 800 nanoseconds on a 2C/4T platform and 2~3 microseconds on a 2S/36C/72T Skylake server in normal case, and in worst case where vm_committed_as's spinlock is under severe contention, it costs 30~40 microseconds for the 2S/36C/72T Skylake sever. Thanks, Feng > > int overcommit_policy_handler(struct ctl_table *table, int write, void *buffer, > > size_t *lenp, loff_t *ppos) > > { > > int ret; > > > > ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); > > - if (ret == 0 && write) > > + if (ret == 0 && write) { > > + if (sysctl_overcommit_memory == OVERCOMMIT_NEVER) > > + schedule_on_each_cpu(sync_overcommit_as); > > + > > mm_compute_batch(); > > + } > > > > return ret; > > } > > -- > > 2.7.4
