RE: [PATCH] perf_counter: Fix a race on perf_counter_ctx

[Metzger, Markus T]

>-----Original Message-----
>From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
>Sent: Tuesday, August 18, 2009 4:02 PM
>To: Metzger, Markus T
>Cc: Ingo Molnar; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx; markus.t.metzger@xxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx; Paul Mackerras
>Subject: RE: [PATCH] perf_counter: Fix a race on perf_counter_ctx
>
>On Tue, 2009-08-18 at 14:54 +0100, Metzger, Markus T wrote:
>
>> Well, that would push out the limit a bit, but it would still be quite fragile.
>>
>> Currently, I'm not sure that this (i.e. that the interrupt handling takes too long)
>> is the underlying problem of the hangs that I'm seeing.
>>
>> If it truly is, then I would go with the two-buffer approach. This would make the
>> ISR itself predictably and reliably fast - at the expense of additional locked memory.
>
>comment out that perf_counter_output() call and see what happens ;-)

That's what I did more or less.
When I ignore the buffer size and sample 10 records, it seems to work.
When I sample 1000 records, the kernel hangs.

But this does not automatically mean that the increased time in the ISR is
directly causing the hang, does it?


>> Do you think that this truly is the problem?
>> How would the kernel react if interrupts were disabled for too long? I would definitely
>> expect bad responsiveness, but can the kernel kill itself?
>
>If we're branch tracing while processing the interrupt (not sure here),
>then we might well generate enough output to generate a new interrupt,
>causing back-to-back interrupts, basically DoSing the machine.

We call perf_disable() when entering the ISR and perf_enable() when exiting.

regards,
markus.

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