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Commit 4ef11014 authored by Jeremy Kerr's avatar Jeremy Kerr
Browse files

[POWERPC] spufs: fix scheduler starvation by idle contexts 



2.6.25 has a regression where we can starve the scheduler by creating
(N_SPES+1) contexts, then running them one at a time.

The final context will never be run, as the other contexts are loaded on
the SPEs, none of which are repoted as free (ie, spu->alloc_state !=
SPU_FREE), so spu_get_idle() doesn't give us a spu to run on. Because
all of the contexts are stopped, none are descheduled by the scheduler
tick, as spusched_tick returns if spu_stopped(ctx).

This change replaces the spu_stopped() check with checking for SCHED_IDLE
in ctx->policy. We set a context's policy to SCHED_IDLE when we're not
in spu_run(). We also favour SCHED_IDLE contexts when looking for contexts
to unbind, but leave their timeslice intact for later resumption.

This patch fixes the following test in the spufs-testsuite:
  tests/20-scheduler/02-yield-starvation

Signed-off-by: default avatarJeremy Kerr <jk@ozlabs.org>
parent 101142c3

arch/powerpc/platforms/cell/spufs/run.c

+1 −0
Original line number Diff line number Diff line
@@ -234,6 +234,7 @@ static int spu_run_fini(struct spu_context *ctx, u32 *npc, 
	*npc = ctx->ops->npc_read(ctx);



	spuctx_switch_state(ctx, SPU_UTIL_IDLE_LOADED);

	ctx->policy = SCHED_IDLE;

	spu_release(ctx);



	if (signal_pending(current))

arch/powerpc/platforms/cell/spufs/sched.c

+3 −5
Original line number Diff line number Diff line
@@ -856,21 +856,18 @@ static noinline void spusched_tick(struct spu_context *ctx) 
{

	struct spu_context *new = NULL;

	struct spu *spu = NULL;

	u32 status;



	if (spu_acquire(ctx))

		BUG();	/* a kernel thread never has signals pending */



	if (ctx->state != SPU_STATE_RUNNABLE)

		goto out;

	if (spu_stopped(ctx, &status))

		goto out;

	if (ctx->flags & SPU_CREATE_NOSCHED)

		goto out;

	if (ctx->policy == SCHED_FIFO)

		goto out;



	if (--ctx->time_slice)

	if (--ctx->time_slice && ctx->policy != SCHED_IDLE)

		goto out;



	spu = ctx->spu;
@@ -880,6 +877,7 @@ static noinline void spusched_tick(struct spu_context *ctx) 
	new = grab_runnable_context(ctx->prio + 1, spu->node);

	if (new) {

		spu_unschedule(spu, ctx);

		if (ctx->policy != SCHED_IDLE)

			spu_add_to_rq(ctx);

	} else {

		spu_context_nospu_trace(spusched_tick__newslice, ctx);