linux-kernel - RE: [PATCH RFC V4 5/6] perf top: switch to backward overwrite mode

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Date:   Mon, 2 Oct 2017 17:19:41 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Jiri Olsa <jolsa@...hat.com>
CC:     "acme@...nel.org" <acme@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "Odzioba, Lukasz" <lukasz.odzioba@...el.com>,
        "wangnan0@...wei.com" <wangnan0@...wei.com>,
        "hekuang@...wei.com" <hekuang@...wei.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: RE: [PATCH RFC V4 5/6] perf top: switch to backward overwrite mode

> On Fri, Sep 29, 2017 at 07:47:56AM -0700, kan.liang@...el.com wrote:
> > From: Kan Liang <kan.liang@...el.com>
> >
> > perf_top__mmap_read has severe performance issue in Knights
> > Landing/Mill, when monitoring in heavy load system. It costs several
> > minutes to finish, which is unacceptable.
> >
> > perf top was overwrite mode. But it is changed to non overwrite mode
> > since commit 93fc64f14472 ("perf top: Switch to non overwrite mode").
> > For non overwrite mode, it tries to read everything in the ring buffer
> > and does not check the messup. Once there are lots of samples
> > delivered shortly, the processing time could be very long.
> > Knights Landing/Mill as a manycore processor contains a large number
> > of small cores. Because of the huge core number, it will generated
> > lots of samples in a heavy load system. Also, since the huge sample#,
> > the mmap writer probably bite the tail and mess up the samples.
> >
> > Also, to avoid the problems which is described in commit 9ecda41acb97
> > ("perf/core: Add ::write_backward attribute to perf event"), switch to
> > backward overwrite mode.
> > Pausing the ring-buffer during perf_top__mmap_read to ensure the
> > ring-buffer is stable.
> > There would be some records lost in backward overwrite mode. Removing
> > the lost events checking.
> 
> I'm getting perf top hogging the cpu completely with this change
> 

I think I find the root cause of the cpu hogging.
perf_mmap__read_catchup discards the md->prev from previous mmap_read.
Current mmap_read doesn't know which data has already been processed by
previous mmap_read. So it has to go through all the valid data in the ring buffer,
even most of the data has been processed by previous mmap_read.

Also, it looks perf record has the similar issue.
The previous location will be discarded as well in backward overwrite mode.
That will be an issue when --overwrite and --switch-output are enabled.
The new output will always include the old data in the previous output, which
should be wrong.

I think I will rewrite the perf_mmap__read_backward and perf_mmap__read_catchup
to fix this issue in a separate thread. Those functions should be common backward
mmap_read functions for all tools and tests.

BTW, are you OK with patch 1-4?
Those patches multithreading the machine__synthesize_threads, which is
irrelevant with the overwrite mode.
I think they can be merged separately.

Thanks,
Kan