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Message-ID: <20200706132443.GA34488@shbuild999.sh.intel.com>
Date: Mon, 6 Jul 2020 21:24:43 +0800
From: Feng Tang <feng.tang@...el.com>
To: Qian Cai <cai@....pw>, Andrew Morton <akpm@...ux-foundation.org>,
Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
Christoph Lameter <cl@...ux.com>
Cc: kernel test robot <rong.a.chen@...el.com>,
Michal Hocko <mhocko@...e.com>,
Johannes Weiner <hannes@...xchg.org>,
Matthew Wilcox <willy@...radead.org>,
Mel Gorman <mgorman@...e.de>,
Kees Cook <keescook@...omium.org>,
Luis Chamberlain <mcgrof@...nel.org>,
Iurii Zaikin <yzaikin@...gle.com>, andi.kleen@...el.com,
tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org, lkp@...ts.01.org
Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail
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@...el.com>
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@intel.com/
[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@...el.com>
Signed-off-by: Feng Tang <feng.tang@...el.com>
---
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.
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