Date: Sun, 23 Jun 2013 01:10:00 GMT From: hiren panchasara <hiren.panchasara@gmail.com> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/179827: [hwpmc] process-mode counters aren't correctly read on multi-core machines Message-ID: <201306230110.r5N1A0eM050747@freefall.freebsd.org>
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The following reply was made to PR kern/179827; it has been noted by GNATS.
From: hiren panchasara <hiren.panchasara@gmail.com>
To: Adrian Chadd <adrian@freebsd.org>
Cc: bug-followup@freebsd.org
Subject: Re: kern/179827: [hwpmc] process-mode counters aren't correctly
read on multi-core machines
Date: Sat, 22 Jun 2013 18:05:13 -0700
On Sat, Jun 22, 2013 at 1:23 PM, Adrian Chadd <adrian@freebsd.org> wrote:
> Right. Do that but with the test running in another window, so you
> don't get the message overlap.
Here is how it looks:
# p/instructions
293898819
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
0
# p/instructions
201677536862
>
> THen look at how the results show lots of '0' from pmcstat, and then
> correlate that with the one-second "PMC,OPS" sample lines. See how
> whenever they're run whilst the process is running (ie, the last event
> was SWI) and it hasn't yet been de-scheduled. because it's still
> running, the saved counter is never updated with the PMC counter and
> subsequent reads (until it does get de-scheduled!) return the same
> cached value. Hence, lots of '0's, followed by a big, big counter
> value.
Precisely how it looks.
Thanks for the explanation.
cheers,
Hiren
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