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); +} + 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 On Sun, Jul 05, 2020 at 10:36:14PM -0400, Qian Cai wrote: > > In my last email, I was not saying OVERCOMMIT_NEVER is not a normal case, > > but I don't think user will too frequently runtime change the overcommit > > policy. And the fix patch of syncing 'vm_committed_as' is only called when > > user calls 'sysctl -w vm.overcommit_memory=2'. > > > > > The question is now if any of those regression fixes would now regress > > > performance of OVERCOMMIT_NEVER workloads or just in-par with the data > > > before the patchset? > > > > For the original patchset, it keeps vm_committed_as unchanged for > > OVERCOMMIT_NEVER policy and enlarge it for the other 2 loose policies > > OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS, and I don't expect the "OVERCOMMIT_NEVER > > workloads" performance will be impacted. If you have suggetions for this > > kind of benchmarks, I can test them to better verify the patchset, thanks! > > Then, please capture those information into a proper commit log when you > submit the regression fix on top of the patchset, and CC PER-CPU MEMORY > ALLOCATOR maintainers, so they might be able to review it properly.
