linux-kernel - Re: [PATCH v2 11/12] perf/x86: make HT bug workaround conditioned on HT enabled

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Date:	Wed, 22 Oct 2014 18:42:16 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"Liang, Kan" <kan.liang@...el.com>, Borislav Petkov <bp@...en8.de>,
	Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
Subject: Re: [PATCH v2 11/12] perf/x86: make HT bug workaround conditioned on
 HT enabled

On Wed, Oct 22, 2014 at 4:58 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> On Thu, Oct 09, 2014 at 06:34:45PM +0200, Stephane Eranian wrote:
>
> SNIP
>
>> --- a/arch/x86/kernel/cpu/perf_event_intel.c
>> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
>> @@ -12,6 +12,7 @@
>>  #include <linux/init.h>
>>  #include <linux/slab.h>
>>  #include <linux/export.h>
>> +#include <linux/watchdog.h>
>>
>>  #include <asm/cpufeature.h>
>>  #include <asm/hardirq.h>
>> @@ -1811,7 +1812,7 @@ intel_start_scheduling(struct cpu_hw_events *cpuc)
>>       /*
>>        * nothing needed if in group validation mode
>>        */
>> -     if (cpuc->is_fake)
>> +     if (cpuc->is_fake || !is_ht_workaround_enabled())
>>               return;
>>       /*
>>        * no exclusion needed
>> @@ -1849,8 +1850,9 @@ intel_stop_scheduling(struct cpu_hw_events *cpuc)
>>       /*
>>        * nothing needed if in group validation mode
>>        */
>> -     if (cpuc->is_fake)
>> +     if (cpuc->is_fake || !is_ht_workaround_enabled())
>>               return;
>
>
> Is it possible to change is_ht_workaround_enabled() result within
> intel_start_scheduling and intel_stop_scheduling time frame? like
> following would happen:
>
>
>   is_ht_workaround_enabled == 1
>
>   intel_start_scheduling -> spin_lock(&excl_cntrs->lock);
>
>   # echo 0 > /sys/devices/cpu/ht_bug_workaround
>
>   is_ht_workaround_enabled == 0
>
>   intel_stop_scheduling -> wont call spin_unlock(&excl_cntrs->lock)
>
>
> for some reason I can't get HT enabled on my HSW server, so I cannot try ;-)
>
Yes, that would be possible!
I can fix this by grabbing the lock in the sysctl handler.
Thanks.
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