Re: [PATCH 09/11] x86, perf: Support sysfs files depending on SMT status
From: Peter Zijlstra
Date: Mon Jan 18 2016 - 13:06:25 EST
On Fri, Jan 15, 2016 at 05:12:51PM -0800, Andi Kleen wrote:
> +ssize_t events_ht_sysfs_show(struct device *dev, struct device_attribute *attr,
> + char *page)
> + struct perf_pmu_events_ht_attr *pmu_attr =
> + container_of(attr, struct perf_pmu_events_ht_attr, attr);
> + /*
> + * Report conditional events depending on Hyper-Threading.
> + *
> + * This is overly conservative as usually the HT special
> + * handling is not needed if the other CPU thread is idle.
> + *
> + * Note this does not (cannot) handle the case when thread
> + * siblings are invisible, for example with virtualization
> + * if they are owned by some other guest. The user tool
> + * has to re-read when a thread sibling gets onlined later.
> + */
> + return sprintf(page, "%s",
> + x86_pmu.ht_on ?
> + pmu_attr->event_str_ht :
> + pmu_attr->event_str_noht);
So one obvious problem with this is that a concurrent hotplug operation
can toggle ht_on resulting in bogus measurements.
I'm not sure there's anything we can do about that, other than WARN in
the tool if it finds inconsistent results; does it?