linux-kernel - Re: [PATCH] perf powerpc: Don't call perf_event

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Date:   Tue, 4 Oct 2016 09:06:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Jiri Olsa <jolsa@...hat.com>, Michael Neuling <mikey@...ling.org>,
        Paul Mackerras <paulus@...ba.org>,
        Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jan Stancek <jstancek@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] perf powerpc: Don't call perf_event_disable from atomic
 context

On Tue, Oct 04, 2016 at 03:29:33PM +1100, Michael Ellerman wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > So it would be good to also explain why PPC needs this in the first
> > place.
> 
> Unfortunately I don't really know the code, and the original author is AWOL.
> 
> But AFAICS perf_event_disable() is only called here:
> 
> 	if (!stepped) {
> 		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
> 			"0x%lx will be disabled.", info->address);
> 		perf_event_disable(bp);
> 		goto out;
> 	}
> 
> Which is where we cope with the possibility that we couldn't emulate the
> instruction that hit the breakpoint. Seems that is not an issue on x86,
> or it's handled elsewhere?

I don't think x86 ever needs to emulate things on hw breakpoint
(although I could be mistaken), but I would expect ARM to maybe need
so, and I couldn't find a disable there either.

Will?

> We should fix emulate_step() if it failed to emulate something it
> should have, but there will always be the possibility that it fails.
> 
> Instead of calling perf_event_disable() we could just add a flag to
> arch_hw_breakpoint that says we hit an error on the event, and block
> reinstalling it in arch_install_hw_breakpoint().

Possible..