Re: [Cbe-oss-dev] [RFC, PATCH 3/4] Add support to OProfile for profiling Cell BE SPUs -- update

[Maynard Johnson <maynardj@us.ibm.com>]

Arnd Bergmann wrote:

> On Monday 29 January 2007 20:48, Maynard Johnson wrote:
>
> > Subject: Enable SPU switch notification to detect currently active SPU tasks.
> >
> > From: Maynard Johnson <maynardj@us.ibm.com>
> >
> > This patch adds to the capability of spu_switch_event_register so that the
> > caller is also notified of currently active SPU tasks.  It also exports
> > spu_switch_event_register and spu_switch_event_unregister.
> >
> > Signed-off-by: Maynard Johnson <mpjohn@us.ibm.com>
>
>
> I looked through it again, and think I found a serious bug, but that
> should be easy enough to solve:
>
>
> > +static void notify_spus_active(void)
> > +{
> > +       int node;
> > +       /* Wake up the active spu_contexts. When the awakened processes 
> > +        * sees their notify_active flag is set, they will call
> > +        * spu_switch_notify();
> > +        */
> > +       for (node = 0; node < MAX_NUMNODES; node++) {
> > +               struct spu *spu;
> > +               mutex_lock(&spu_prio->active_mutex[node]);
> > +                list_for_each_entry(spu, &spu_prio->active_list[node], list) {
> > +                       struct spu_context *ctx = spu->ctx;
>
>
> [side note]
> There is a small whitespace breakage in here, please make sure you always
> use tabs for indenting, not space characters.
> [/side note]
>
>
> > @@ -45,9 +45,10 @@
> >        u64 pte_fault;
> >
> >        *stat = ctx->ops->status_read(ctx);
> > -       if (ctx->state != SPU_STATE_RUNNABLE)
> > -               return 1;
> > +
> >        spu = ctx->spu;
> > +       if (ctx->state != SPU_STATE_RUNNABLE || spu->notify_active)
> > +               return 1;
> >        pte_fault = spu->dsisr &
> >            (MFC_DSISR_PTE_NOT_FOUND | MFC_DSISR_ACCESS_DENIED);
> >        return (!(*stat & 0x1) || pte_fault || spu->class_0_pending) ? 1 : 0;
> > @@ -305,6 +306,7 @@
> >                   u32 *npc, u32 *event)
> > {
> >        int ret;
> > +       struct spu * spu;
> >        u32 status;
> >
> >        if (down_interruptible(&ctx->run_sema))
> > @@ -318,8 +320,16 @@
> >
> >        do {
> >                ret = spufs_wait(ctx->stop_wq, spu_stopped(ctx, &status));
> > +               spu = ctx->spu;
> >                if (unlikely(ret))
> >                        break;
> > +               if (unlikely(spu->notify_active)) {
> > +                       spu->notify_active = 0;
> > +                       if (!(status & SPU_STATUS_STOPPED_BY_STOP)) {
> > +                               spu_switch_notify(spu, ctx);
> > +                               continue;
> > +                       }
> > +               }
>
>
> This is before spu_reacquire_runnable, so in case the spu got
> preempted at the same time when oprofile was enabled, ctx->spu
> is NULL, and you can't load the notify_active flag from it.
>
> On solution would be to move the notify_active flag from ctx->spu
> into ctx itself, but maybe there are other ways to solve this.
In an earlier review of this patch, Christopher Hellwig suggested I move 
the notify_active flag to be a bit in the sched_flags field that's added 
in his scheduler patch series.  If this patch series will be a available 
in an "Arnd" tree that we'll be using for our current OProfile 
development, perhaps I should wait until that time to change this, since 
the window of vulnerability is quite small.  What do you think?

-Maynard
> Thanks,
>
> 	Arnd <><


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