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