linux-kernel - Re: [PATCH v6 2/2] ipc/msg: mitigate the lock contention with percpu counter

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Date:   Tue, 20 Sep 2022 13:50:20 +0800
From:   "Sun, Jiebin" <jiebin.sun@...el.com>
To:     Manfred Spraul <manfred@...orfullife.com>,
        akpm@...ux-foundation.org, vasily.averin@...ux.dev,
        shakeelb@...gle.com, dennis@...nel.org, tj@...nel.org,
        cl@...ux.com, ebiederm@...ssion.com, legion@...nel.org,
        alexander.mikhalitsyn@...tuozzo.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     tim.c.chen@...el.com, feng.tang@...el.com, ying.huang@...el.com,
        tianyou.li@...el.com, wangyang.guo@...el.com,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v6 2/2] ipc/msg: mitigate the lock contention with percpu
 counter


On 9/20/2022 12:53 PM, Manfred Spraul wrote:
> On 9/20/22 04:36, Sun, Jiebin wrote:
>>
>> On 9/18/2022 8:53 PM, Manfred Spraul wrote:
>>> 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@...el.com>
>>>> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
>>> Reviewed-by: Manfred Spraul <manfred@...orfullif.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.
>>>
>> Hi Manfred,
>>
>> Thanks for your advice. But I'm not sure if we could fix the overflow 
>> issue in ipc/msg totally by
>>
>> clamp(val, low, INT_MAX). If the value is over s32, we might avoid 
>> the reversal sign, but still could
>>
>> not get the accurate value.
>
> I think just clamping it to INT_MAX is the best approach.
> Reporting negative values is worse than clamping. If (and only if) 
> there are real users that need to know the total amount of memory 
> allocated for messages queues in one namespace, then we could add a 
> MSG_INFO64 with long values. But I would not add that right now, I do 
> not see a real use case where the value would be needed.
>
> Any other opinions?
>
> -- 
>
>     Manfred
>
>
OK. I will work on it and send it out for review.