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Date: Mon, 28 Jun 2010 17:29:26 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: paulus <paulus@samba.org>, stephane eranian <eranian@googlemail.com>,
       Robert Richter <robert.richter@amd.com>,
       Will Deacon <will.deacon@arm.com>, Paul Mundt <lethal@linux-sh.org>,
       Cyrill Gorcunov <gorcunov@gmail.com>, Lin Ming <ming.m.lin@intel.com>,
       Yanmin <yanmin_zhang@linux.intel.com>,
       Deng-Cheng Zhu <dengcheng.zhu@gmail.com>,
       David Miller <davem@davemloft.net>, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 05/11] perf: register pmu implementations
Message-ID: <20100628152924.GA5492@nowhere>
References: <20100624142804.431553874@chello.nl> <20100624143406.764162308@chello.nl> <20100628132101.GB5668@nowhere> <1277738197.3561.132.camel@laptop>
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On Mon, Jun 28, 2010 at 05:16:37PM +0200, Peter Zijlstra wrote:
> > >  	}
> > > +	srcu_read_unlock(&pmus_srcu, idx);
> > >  
> > >  	return pmu;
> > >  }
> > 
> > 
> > 
> > I'm still not sure why all this locking is needed. We don't even
> > support pmus in modules.
> > 
> > Is there something coming soon that will use this?
> > I remember something about KVM.
> 
> Possibly, not sure. We could put the unregister thing in a later patch,
> but I wanted to make sure it was sanely possibly and its only a few
> lines of code.


Ok.

 
> > And who will have to use srcu? It seems the event fastpath would
> > be concerned, right? Will that have an impact on the performances?
> 
> Only event creation like above (perf_init_event) will have to use SRCU,
> so not really a hot path.


Ah I see. The event itself is synchronized against the fast-path using rcu.
And then pmus themselves would be synchronized against events. Right
that makes sense.

But then why RCU (or SRCU, whatever)? I mean parent event creation is
quite rare. And child events won't need to be synchronized as far as the parent
keeps a reference to the pmu.

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