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

[Sun, Jiebin]

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@xxxxxxxxx>
> > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Reviewed-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxx>
> > @@ -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.