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

Thanks,

Arnd <><
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