linux-kernel - Re: [PATCH V7 3/6] perf, x86: handle multiple records in PEBS buffer

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Message-ID: <20150506131315.GA23123@twins.programming.kicks-ass.net>
Date:	Wed, 6 May 2015 15:13:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Liang, Kan" <kan.liang@...el.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"acme@...radead.org" <acme@...radead.org>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V7 3/6] perf, x86: handle multiple records in PEBS buffer

On Wed, May 06, 2015 at 03:01:16PM +0200, Andi Kleen wrote:
> > > > +		if (p->status != (1 << bit)) {
> > > > +			u64 pebs_status;
> > > > +
> > > > +			/* slow path */
> > > > +			pebs_status = p->status & cpuc->pebs_enabled;
> > > > +			pebs_status &= (1ULL << MAX_PEBS_EVENTS) - 1;
> > > > +			if (pebs_status != (1 << bit)) {
> > > > +				perf_log_lost(event);
> > > 
> > > Does it make sense to keep an error[bit] count and only log once with the
> > > actual number in? -- when !0 obviously.
> > 
> > Yes, will do it.
> 
> If you use anything but u8 for the array member it would be too large
> for the NMI stack, and u8 is lilkely overflow prone. Would not do it.

If we're so close that 4*8=32 bytes would overflow the stack we're in
trouble already.

But we could just switch counts[] over to short and have another short
array for errors[], that way no extra bytes of stack used.
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