linux-kernel - Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu counter

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Date:   Thu, 8 Sep 2022 08:38:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Sun, Jiebin" <jiebin.sun@...el.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>, vasily.averin@...ux.dev,
        shakeelb@...gle.com, dennis@...nel.org, tj@...nel.org,
        cl@...ux.com, ebiederm@...ssion.com, legion@...nel.org,
        manfred@...orfullife.com, alexander.mikhalitsyn@...tuozzo.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...el.com, feng.tang@...el.com, ying.huang@...el.com,
        tianyou.li@...el.com, wangyang.guo@...el.com
Subject: Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu
 counter

On Thu, 8 Sep 2022 16:25:47 +0800 "Sun, Jiebin" <jiebin.sun@...el.com> wrote:

> In our case, if the local 
> percpu counter is near to INT_MAX and there comes a big msgsz, the 
> overflow issue could happen.

percpu_counter_add_batch() handles this - your big message
won't overflow an s64.


Lookng at percpu_counter_add_batch(), is this tweak right?

- don't need to update *fbc->counters inside the lock
- that __this_cpu_sub() is an obscure way of zeroing the thing

--- a/lib/percpu_counter.c~a
+++ a/lib/percpu_counter.c
@@ -89,8 +89,8 @@ void percpu_counter_add_batch(struct per
 		unsigned long flags;
 		raw_spin_lock_irqsave(&fbc->lock, flags);
 		fbc->count += count;
-		__this_cpu_sub(*fbc->counters, count - amount);
 		raw_spin_unlock_irqrestore(&fbc->lock, flags);
+		__this_cpu_write(*fbc->counters, 0);
 	} else {
 		this_cpu_add(*fbc->counters, amount);
 	}
_