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Date:   Wed, 7 Sep 2022 14:34:27 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     Jiebin Sun <jiebin.sun@intel.com>, vasily.averin@linux.dev,
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        tim.c.chen@intel.com, feng.tang@intel.com, ying.huang@intel.com,
        tianyou.li@intel.com, wangyang.guo@intel.com
Subject: Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu
 counter
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On Wed, 07 Sep 2022 09:01:53 -0700 Tim Chen <tim.c.chen@linux.intel.com> wrote:

> On Thu, 2022-09-08 at 01:25 +0800, Jiebin Sun wrote:
> > The msg_bytes and msg_hdrs atomic counters are frequently
> > updated when IPC msg queue is in heavy use, causing heavy
> > cache bounce and overhead. Change them to percpu_counter
> > greatly improve the performance. Since there is one percpu
> > struct per namespace, additional memory cost is minimal.
> > Reading of the count done in msgctl call, which is infrequent.
> > So the need to sum up the counts in each CPU is infrequent.
> > 
> > 
> > Apply the patch and test the pts/stress-ng-1.4.0
> > -- system v message passing (160 threads).
> > 
> > Score gain: 3.17x
> > 
> > 
> ...
> >  
> > +/* large batch size could reduce the times to sum up percpu counter */
> > +#define MSG_PERCPU_COUNTER_BATCH 1024
> > +
> 
> Jiebin, 
> 
> 1024 is a small size (1/4 page). 
> The local per cpu counter could overflow to the gloabal count quickly
> if it is limited to this size, since our count tracks msg size.
>   
> I'll suggest something larger, say 8*1024*1024, about
> 8MB to accommodate about 2 large page worth of data.  Maybe that
> will further improve throughput on stress-ng by reducing contention
> on adding to the global count.
> 

I think this concept of a percpu_counter_add() which is massively
biased to the write side and with very rare reading is a legitimate
use-case.  Perhaps it should become an addition to the formal interface.
Something like

/* 
 * comment goes here
 */
static inline void percpu_counter_add_local(struct percpu_counter *fbc,
					    s64 amount)
{
	percpu_counter_add_batch(fbc, amount, INT_MAX);
}

and percpu_counter_sub_local(), I guess.

The only instance I can see is
block/blk-cgroup-rwstat.h:blkg_rwstat_add() which is using INT_MAX/2
because it always uses percpu_counter_sum_positive() on the read side.

But that makes two!