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

[David Ahern]

On 9/15/14, 6:57 AM, Alexander Yarygin wrote:
> David Ahern <dsahern@xxxxxxxxx> 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...

No, you understand the problem. My point is that the key issue with the 
code (overwriting events in the mmap'ed buffers) really has nothing to 
do with perf-kvm. It might be the only command in-tree today but others 
could come along, so might as well fix the issue in the event processing 
code.

> 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? :)

Sure. With nested VMs on x86 I see 100,000's of events per second. That 
use case is what drove me to look at copying events. I just have not had 
time to come back to that one.

David

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