linux-kernel - Re: [PATCH RFC] perf kvm stat live: cache mmap()ed events

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Date:	Mon, 15 Sep 2014 16:57:34 +0400
From:	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>
To:	David Ahern <dsahern@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC] perf kvm stat live: cache mmap()ed events

David Ahern <dsahern@...il.com> writes:

> On 9/12/14, 10:27 AM, Alexander Yarygin wrote:
>> During mmap() process 'perf kvm stat live' gets a pointer to events and
>> passes them to the session queue. Events are stored in shared memory and
>> eventually they will be overwritten by the kernel. The problem is, that
>> when events come too fast, old events can be overwritten before they
>> have been processed that can lead to perf crash.
>>
>> To prevent that happening, we can copy upcoming events and pass a copy
>> to the session queue. There is a safe place to copy event: before
>> perf_evlist__mmap_consume() is executed. There are 3 places to free it:
>> when event is processed, when it's lost and on exit, if it's turned out
>> unprocessed.
>
> Did you see what I proposed a year ago:
> https://lkml.org/lkml/2013/9/6/388
>
> The intent is to keep the copy generic and not local a command since
> conceptually other live commands need the same.
>
> David

Hello,

Yes, your patch works fine. But as far as I understand, right now only
the 'perf kvm stat live' is infected: other 'live' tools were fixed by
the patch "PERF: The tail position of the event buffer should only be modified
after actually use that event." and since they don't use ordered queue
they don't need a copying. That's why I came up with 'perf kvm stat
live' specific approach. Maybe I missed something...

Anyhow, having >30K events is a quite usual situation on s390 and the
'perf kvm stat live' command hardly works there, so it would be good to
have at least some working solution. Any ideas? :)

Thanks.

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