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

["Sun, Jiebin" <jiebin.sun@intel.com>]

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@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.
>
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.