Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 8 Sep 2022 08:38:59 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     "Sun, Jiebin" <jiebin.sun@intel.com>
Cc:     Tim Chen <tim.c.chen@linux.intel.com>, vasily.averin@linux.dev,
        shakeelb@google.com, dennis@kernel.org, tj@kernel.org,
        cl@linux.com, ebiederm@xmission.com, legion@kernel.org,
        manfred@colorfullife.com, alexander.mikhalitsyn@virtuozzo.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        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
Message-Id: <20220908083859.24c989f08d62ddbd031005de@linux-foundation.org>
In-Reply-To: <c8e771c8-4b01-f2b4-5b54-e9931f556270@intel.com>
References: <CALvZod44uUFnwfF4StC24t+d1s_XE10hkmSCgb04FjtTATo6xQ@mail.gmail.com>
        <20220907172516.1210842-1-jiebin.sun@intel.com>
        <eb13cb3e5e1625afe1bb783810f4d6b52a66a2f6.camel@linux.intel.com>
        <20220907143427.0ce54bbf096943ffca197fee@linux-foundation.org>
        <c8e771c8-4b01-f2b4-5b54-e9931f556270@intel.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Thu, 8 Sep 2022 16:25:47 +0800 "Sun, Jiebin" <jiebin.sun@intel.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);
 	}
_