Re: [PATCH RFC V4 5/6] perf top: switch to backward overwrite mode

[Jiri Olsa]

On Mon, Oct 02, 2017 at 05:19:41PM +0000, Liang, Kan wrote:
> 
> 
> > On Fri, Sep 29, 2017 at 07:47:56AM -0700, kan.liang@xxxxxxxxx wrote:
> > > From: Kan Liang <kan.liang@xxxxxxxxx>
> > >
> > > 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.

yep, for patches 1-4:

Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>

thanks,
jirka