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Message-ID: <aadf6c7e-dea8-4dff-1815-cca9c2c2da9e@colorfullife.com>
Date:   Sun, 18 Sep 2022 14:53:48 +0200
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Subject: Re: [PATCH v6 2/2] ipc/msg: mitigate the lock contention with percpu
 counter
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To:     Jiebin Sun <jiebin.sun@intel.com>, akpm@linux-foundation.org,
        vasily.averin@linux.dev, shakeelb@google.com, dennis@kernel.org,
        tj@kernel.org, cl@linux.com, ebiederm@xmission.com,
        legion@kernel.org, alexander.mikhalitsyn@virtuozzo.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc:     tim.c.chen@intel.com, feng.tang@intel.com, ying.huang@intel.com,
        tianyou.li@intel.com, wangyang.guo@intel.com,
        Tim Chen <tim.c.chen@linux.intel.com>
References: <20220902152243.479592-1-jiebin.sun@intel.com>
 <20220913192538.3023708-1-jiebin.sun@intel.com>
 <20220913192538.3023708-3-jiebin.sun@intel.com>
From:   Manfred Spraul <manfred@colorfullife.com>
In-Reply-To: <20220913192538.3023708-3-jiebin.sun@intel.com>
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Hi Jiebin,

On 9/13/22 21:25, 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: 3.99x
>
> CPU: ICX 8380 x 2 sockets
> Core number: 40 x 2 physical cores
> Benchmark: pts/stress-ng-1.4.0
> -- system v message passing (160 threads)
>
> Signed-off-by: Jiebin Sun <jiebin.sun@intel.com>
> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Manfred Spraul <manfred@colorfullif.com>
> @@ -495,17 +496,18 @@ static int msgctl_info(struct ipc_namespace *ns, int msqid,
>   	msginfo->msgssz = MSGSSZ;
>   	msginfo->msgseg = MSGSEG;
>   	down_read(&msg_ids(ns).rwsem);
> -	if (cmd == MSG_INFO) {
> +	if (cmd == MSG_INFO)
>   		msginfo->msgpool = msg_ids(ns).in_use;
> -		msginfo->msgmap = atomic_read(&ns->msg_hdrs);
> -		msginfo->msgtql = atomic_read(&ns->msg_bytes);
> +	max_idx = ipc_get_maxidx(&msg_ids(ns));
> +	up_read(&msg_ids(ns).rwsem);
> +	if (cmd == MSG_INFO) {
> +		msginfo->msgmap = percpu_counter_sum(&ns->percpu_msg_hdrs);
> +		msginfo->msgtql = percpu_counter_sum(&ns->percpu_msg_bytes);

Not caused by your change, it just now becomes obvious:

msginfo->msgmap and ->msgtql are type int, i.e. signed 32-bit, and the 
actual counters are 64-bit.
This can overflow - and I think the code should handle this. Just clamp 
the values to INT_MAX.

-- 

     Manfred