linux-kernel - Re: [PATCHES/RFC] Re: A concern about overflow ring buffer mode

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Message-ID: <20181029143506.GF21857@kernel.org>
Date:   Mon, 29 Oct 2018 11:35:06 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
        Wang Nan <wangnan0@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Kan Liang <kan.liang@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCHES/RFC] Re: A concern about overflow ring buffer mode

Em Mon, Oct 29, 2018 at 10:33:06AM -0400, Liang, Kan escreveu:
> On 10/29/2018 9:03 AM, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Oct 26, 2018 at 04:11:51PM -0400, Liang, Kan escreveu:
> > > On 10/26/2018 3:24 PM, Arnaldo Carvalho de Melo wrote:
> > > > Em Fri, Oct 26, 2018 at 03:16:29PM -0400, Liang, Kan escreveu:
> > > It is mainly for performance reason to switch to overwrite mode. The impact
> > > was very small when I did my test. But now the effect is easily noticeable
> > > in other tests. Yes, I agree. We may change it back to non-overwrite mode
> > > until the issue is addressed.

> > So, I have these two patches in my perf/core branch, with Fixes tags
> > that will make them get to the stable kernels, ok?
 
> I just realized that the problem in KNL will be back if we switch back to
> non-overwrite mode.
> The problem is that users have to wait tens of minutes to see perf top
> results on the screen in KNL. Before that, there is nothing but a black
> screen.
 
> Sorry I didn't notice it last Friday. Because I thought the ui_warning in
> perf_top__mmap_read() can give user a hint. So the user can switch to
> overwrite mode manually.
> But unfortunately, the ui_warning doesn't work. Because it is called after
> perf_top__mmap_read(). The processing time of perf_top__mmap_read() could be
> tens of minutes.

So we need a way to notice that we're in a machine like that and warn
the user before the wait takes place, ideas on how to do that?

- Arnaldo