linux-kernel - [PATCH] perf kvm record/report: 'unprocessable sample' error while recording/reporting guest data

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Date:	Mon,  7 Dec 2015 12:25:02 +0530
From:	Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
To:	acme@...nel.org
Cc:	linux-kernel@...r.kernel.org, naveen.n.rao@...ux.vnet.ibm.com,
	Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Subject: [PATCH] perf kvm record/report: 'unprocessable sample' error while recording/reporting guest data

While recording guest samples in host using perf kvm record, it will
populate unprocessable sample error, though samples will be recorded
properly. While generating report using perf kvm report, no samples will
be processed and same error will populate. We have seen this behaviour
with upstream perf(4.4-rc3) on x86 and ppc64 hardware.

Reason behind this failure is, when it tries to fetch machine from rb_tree
of machines, it fails. As a part of tracing a bug, we figured out that this
code was incorrectly refactored in commit 54245fdc357613633954bfd38cffb71cb9def067
("perf session: Remove wrappers to machines__find")

This patch will change the functionality such that if it can't fetch
machine in first trial, it will create one node of machine and add that to
rb_tree. So next time when it tries to fetch same machine from rb_tree,
it won't fail. Actually it was the case before refactoring of code in
aforementioned commit.

This patch is generated from acme perf/core branch.

Below I've mention an example that demonstrate the behaviour before and
after applying patch.

Before applying patch:
[Note: One needs to run guest before recording data in host]

ravi@...i-bangoria:~$ ./perf kvm record -a
Warning:
5903 unprocessable samples recorded.
Do you have a KVM guest running and not using 'perf kvm'?
[ perf record: Captured and wrote 1.409 MB perf.data.guest (285 samples) ]

ravi@...i-bangoria:~$ ./perf kvm report --stdio
Warning:
5903 unprocessable samples recorded.
Do you have a KVM guest running and not using 'perf kvm'?
# To display the perf.data header info, please use --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 285  of event 'cycles'
# Event count (approx.): 88715406
#
# Overhead  Command  Shared Object  Symbol
# ........  .......  .............  ......
#

#
# (For a higher level overview, try: perf report --sort comm,dso)
#

After applying patch:

ravi@...i-bangoria:~$ ./perf kvm record -a
[ perf record: Captured and wrote 1.188 MB perf.data.guest (17 samples) ]

ravi@...i-bangoria:~$ ./perf kvm report --stdio
# To display the perf.data header info, please use --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 17  of event 'cycles'
# Event count (approx.): 700746
#
# Overhead  Command  Shared Object     Symbol
# ........  .......  ................  ......................
#
    34.19%  :5758    [unknown]         [g] 0xffffffff818682ab
    22.79%  :5758    [unknown]         [g] 0xffffffff812dc7f8
    22.79%  :5758    [unknown]         [g] 0xffffffff818650d0
    14.83%  :5758    [unknown]         [g] 0xffffffff8161a1b6
     2.49%  :5758    [unknown]         [g] 0xffffffff818692bf
     0.48%  :5758    [unknown]         [g] 0xffffffff81869253
     0.05%  :5758    [unknown]         [g] 0xffffffff81869250

Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
---
 tools/perf/util/session.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index c35ffdd..468de95 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -972,7 +972,7 @@ static struct machine *machines__find_for_cpumode(struct machines *machines,

 		machine = machines__find(machines, pid);
 		if (!machine)
-			machine = machines__find(machines, DEFAULT_GUEST_KERNEL_ID);
+			machine = machines__findnew(machines, DEFAULT_GUEST_KERNEL_ID);
 		return machine;
 	}

-- 
2.1.4

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