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

From: Arnd Bergmann <[email-masked]>
Date: Tue, 30 Jan 2007 05:24:32 +0100

On Monday 29 January 2007 20:48, Maynard Johnson wrote:
> Subject: Enable SPU switch notification to detect currently active SPU tasks.
> 
> From: Maynard Johnson <[email-masked]>
> 
> 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 <[email-masked]>

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.

Thanks,

	Arnd <><