linux-kernel - Re: [PATCH] perf: Fix probing for PERF_FLAG_FD

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Date:	Thu, 19 Feb 2015 09:22:13 -0700
From:	David Ahern <david.ahern@...cle.com>
To:	Adrian Hunter <adrian.hunter@...el.com>,
	David Ahern <dsahern@...il.com>, acme@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Fix probing for PERF_FLAG_FD_CLOEXEC flag

On 2/19/15 9:17 AM, Adrian Hunter wrote:
> Yes, I am sorry it is a pain. I don't know why I didn't add a comment
> to the code :-(. Using -1 for the pid is a workaround to avoid gratuitous
> jump label changes. If pid=0 is used and then a system-wide trace is done
> with Intel PT, there will be a jump label change shortly after the tracing
> starts. That means the running code gets changed, but Intel PT decoding
> has to walk the code to reconstruct the trace - so errors result. There
> will always be occasional jump label changes, but this avoids one that
> would otherwise always happen.

I don't understand the response. Why can't pid == getpid() (ie., pid > 
0) be used for this test? pid = -1 and pid = 0 are not needed. With pid 
 > 0 cpu value does not matter so cpu = -1 can be used. Again this is 
just to determine if the kernel supports PERF_FLAG_FD_CLOEXEC. Existence 
of PT should not be involved here.

David
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