Re: [PATCH] perf, x86: Fix Intel shared extra MSR allocation

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2012-06-05 at 15:30 +0200, Peter Zijlstra wrote:

> @@ -1157,14 +1163,14 @@ __intel_shared_reg_get_constraints(struct cpu_hw_events *cpuc,
>  	struct event_constraint *c = &emptyconstraint;
>  	struct er_account *era;
>  	unsigned long flags;
> -	int orig_idx = reg->idx;
> +	int idx = reg->idx;
>  
>  	/* already allocated shared msr */
>  	if (reg->alloc)
>  		return NULL; /* call x86_get_event_constraint() */

I'm afraid that needs to be:

	/*
	 * reg->alloc can be set due to existing state, so for fake cpuc
	 * we need to ignore this, otherwise we might fail to allocate
	 * proper fake state for this extra reg constraint. Also see
	 * the comment below.
	 */
	if (reg->alloc && !cpuc->is_fake)
		return NULL; /* call x86_get_event_constraints() */

>  
>  again:
> -	era = &cpuc->shared_regs->regs[reg->idx];
> +	era = &cpuc->shared_regs->regs[idx];
>  	/*
>  	 * we use spin_lock_irqsave() to avoid lockdep issues when
>  	 * passing a fake cpuc
> @@ -1173,6 +1179,29 @@ __intel_shared_reg_get_constraints(struct cpu_hw_events *cpuc,
>  
>  	if (!atomic_read(&era->ref) || era->config == reg->config) {
>  
> +		/*
> +		 * If its a fake cpuc -- as per validate_{group,event}() we
> +		 * shouldn't touch event state and we can avoid doing so
> +		 * since both will only call get_event_constraints() once
> +		 * on each event, this avoids the need for reg->alloc.
> +		 *
> +		 * Not doing the ER fixup will only result in era->reg being
> +		 * wrong, but since we won't actually try and program hardware
> +		 * this isn't a problem either.
> +		 */
> +		if (!cpuc->is_fake) {
> +			if (idx != reg->idx)
> +				intel_fixup_er(event, idx);
> +
> +			/* 
> +			 * x86_schedule_events() can call get_event_constraints()
> +			 * multiple times on events in the case of incremental
> +			 * scheduling(). reg->alloc ensures we only do the ER
> +			 * allocation once.
> +			 */
> +			reg->alloc = 1;
> +		}
> +
>  		/* lock in msr value */
>  		era->config = reg->config;
>  		era->reg = reg->reg;