From: Michael Ellerman <mpe@ellerman.id.au>
To: Anshuman Khandual <khandual@linux.vnet.ibm.com>, linuxppc-dev@ozlabs.org
Cc: mikey@neuling.org, sukadev@linux.vnet.ibm.com, dja@axtens.net
Subject: Re: [1/5] powerpc/perf: Drop the branch sample when 'from' cannot be fetched
Date: Mon, 27 Jul 2015 14:19:15 +1000 (AEST) [thread overview]
Message-ID: <20150727041915.63106140326@ozlabs.org> (raw)
In-Reply-To: <1435652431-22024-2-git-send-email-khandual@linux.vnet.ibm.com>
On Tue, 2015-30-06 at 08:20:27 UTC, Anshuman Khandual wrote:
> BHRB (Branch History Rolling Buffer) is a rolling buffer. Hence we
> might end up in a situation where we have read one target address
> but when we try to read the next entry indicating the from address
> of the target address, the buffer just overflows. In this case, the
> captured from address will be zero which indicates the end of the
> buffer.
Right. But with SMT8 the size of the buffer is very small, so we will actually
hit this case somewhat often. When we originally wrote this we decided it was
better to get some information, ie. the from address, than no information at
all.
> This patch drops the entire branch record which would have
> otherwise confused the user space tools.
Does it confuse the tools? Can you show me before/after output from perf?
I'm not opposed to changing this but we need to be 100% sure it's the best
option.
cheers
next prev parent reply other threads:[~2015-07-27 4:19 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-30 8:20 [PATCH 0/5] BHRB fixes, improvements and cleanups Anshuman Khandual
2015-06-30 8:20 ` [PATCH 1/5] powerpc/perf: Drop the branch sample when 'from' cannot be fetched Anshuman Khandual
2015-07-27 4:19 ` Michael Ellerman [this message]
2015-07-28 3:08 ` [1/5] " Anshuman Khandual
2015-09-30 9:03 ` Anshuman Khandual
2015-09-30 10:46 ` Michael Ellerman
2015-06-30 8:20 ` [PATCH 2/5] powerpc/perf: Change type of the bhrb_users variable Anshuman Khandual
2015-08-03 1:35 ` [2/5] " Michael Ellerman
2015-06-30 8:20 ` [PATCH 3/5] powerpc/perf: Replace last usage of get_cpu_var with this_cpu_ptr Anshuman Khandual
2015-07-27 5:15 ` [3/5] " Michael Ellerman
2015-07-28 3:37 ` Anshuman Khandual
2015-06-30 8:20 ` [PATCH 4/5] powerpc/perf: Change name & type of 'pred' in power_pmu_bhrb_read Anshuman Khandual
2015-07-29 3:25 ` [4/5] " Michael Ellerman
2015-07-29 8:13 ` Anshuman Khandual
2015-06-30 8:20 ` [PATCH 5/5] powerpc/perf: Re organize PMU branch filter processing on POWER8 Anshuman Khandual
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