linux-kernel - Re: [PATCH v1 2/2] perf header: Support hybrid CPU_PMU

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Date:   Thu, 6 May 2021 15:22:35 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Jin, Yao" <yao.jin@...ux.intel.com>
Cc:     acme@...nel.org, jolsa@...nel.org, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v1 2/2] perf header: Support hybrid CPU_PMU_CAPS

On Thu, May 06, 2021 at 12:59:08PM +0800, Jin, Yao wrote:
> Hi Jiri,
> 
> On 5/4/2021 11:07 PM, Jiri Olsa wrote:
> > On Fri, Apr 30, 2021 at 03:46:02PM +0800, Jin Yao wrote:
> > > On hybrid platform, it may have several cpu pmus, such as,
> > > "cpu_core" and "cpu_atom". The CPU_PMU_CAPS feature in perf
> > > header needs to be improved to support multiple cpu pmus.
> > > 
> > > The new layout in header is defined as:
> > > 
> > > <nr_caps>
> > > <caps string>
> > > <caps string>
> > > <pmu name>
> > > <nr of rest pmus>
> > 
> > not sure why is the 'nr of rest pmus' needed
> > 
> 
> The 'nr of rest pmus' indicates the remaining pmus which are waiting for process.
> 
> For example,
> 
> <nr_caps>
> <caps string>
> "cpu_core"
> 1
> <nr_caps>
> <caps string>
> "cpu_atom"
> 0
> 
> When we see '0' in data file processing, we know all the pmu have been processed yet.
> 
> > the current format is:
> > 
> >          u32 nr_cpu_pmu_caps;
> >          {
> >                  char    name[];
> >                  char    value[];
> >          } [nr_cpu_pmu_caps]
> > 
> > 
> > I guess we could extend it to:
> > 
> >          u32 nr_cpu_pmu_caps;
> >          {
> >                  char    name[];
> >                  char    value[];
> >          } [nr_cpu_pmu_caps]
> > 	char pmu_name[]
> > 
> >          u32 nr_cpu_pmu_caps;
> >          {
> >                  char    name[];
> >                  char    value[];
> >          } [nr_cpu_pmu_caps]
> > 	char pmu_name[]
> > 
> > 	...
> > 
> > and we could detect the old format by checking that there's no
> > pmu name.. but maybe I'm missing something, I did not check deeply,
> > please let me know
> > 
> 
> Actually we do the same thing, but I just add an extra 'nr of rest pmus'
> after the pmu_name. The purpose of 'nr of rest pmus' is when we see '0' at
> 'nr of rest pmus', we know that all pmus have been processed.
> 
> Otherwise, we have to continue reading data file till we find something
> incorrect and then finally drop the last read data.

you have the size of the feature data right? I think we use
it in other cases to check if there are more data

> 
> So is this solution acceptable?
> 
> > also would be great to move the format change and storing hybrid
> > pmus in separate patches
> > 
> 
> Maybe we have to put the storing and processing into one patch.
> 
> Say patch 1 contains the format change and storing hybrid pmus. And patch 2
> contains the processing for the new format. If the repo only contains the
> patch 1, I'm afraid that may introduce the compatible issue.

maybe you can have change of caps format in one patch
and storing/processing hybrid pmus in another?

jirka