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

[Sun, Jiebin]

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@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.
>
> 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.