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

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