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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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