linux-kernel - Re: [PATCH 3/3] perf/x86/intel: force resched when TFA sysctl is modified

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Date:   Fri, 5 Apr 2019 09:13:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        ak@...ux.intel.com, kan.liang@...el.com, mingo@...e.hu,
        nelson.dsouza@...el.com, jolsa@...hat.com, tonyj@...e.com
Subject: Re: [PATCH 3/3] perf/x86/intel: force resched when TFA sysctl is
 modified

On Thu, Apr 04, 2019 at 11:32:19AM -0700, Stephane Eranian wrote:
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index a4b7711ef0ee..8d356c2096bc 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4483,6 +4483,60 @@ static ssize_t freeze_on_smi_store(struct device *cdev,
>  	return count;
>  }
>  
> +static void update_tfa_sched(void *ignored)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	struct pmu *pmu = x86_get_pmu();
> +	struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
> +	struct perf_event_context *task_ctx = cpuctx->task_ctx;
> +
> +	/* prevent any changes to the two contexts */
> +	perf_ctx_lock(cpuctx, task_ctx);
> +
> +	/*
> +	 * check if PMC3 is used
> +	 * and if so force schedule out for all event types all contexts
> +	 */
> +	if (test_bit(3, cpuc->active_mask))
> +		perf_ctx_resched(cpuctx, task_ctx, EVENT_ALL|EVENT_CPU);
> +
> +	perf_ctx_unlock(cpuctx, task_ctx);

I'm not particularly happy with exporting all that. Can't we create this
new perf_ctx_resched() to include the locking and everything. Then the
above reduces to:

	if (test_bit(3, cpuc->active_mask))
		perf_ctx_resched(cpuctx);

And we don't get to export the tricky bits.

> +}
> +
> +static ssize_t show_sysctl_tfa(struct device *cdev,
> +			      struct device_attribute *attr,
> +			      char *buf)
> +{
> +	return snprintf(buf, 40, "%d\n", allow_tsx_force_abort);
> +}
> +
> +static ssize_t set_sysctl_tfa(struct device *cdev,
> +			      struct device_attribute *attr,
> +			      const char *buf, size_t count)
> +{
> +	unsigned long val;
> +	ssize_t ret;
> +
> +	ret = kstrtoul(buf, 0, &val);

You want kstrtobool()

> +	if (ret)
> +		return ret;
> +
> +	/* looking for boolean value */
> +	if (val > 2)
> +		return -EINVAL;
> +
> +	/* no change */
> +	if (val == allow_tsx_force_abort)
> +		return count;
> +
> +	allow_tsx_force_abort ^= 1;

	allow_tsx_force_abort = val;

is simpler

> +
> +	on_each_cpu(update_tfa_sched, NULL, 1);
> +
> +	return count;
> +}