linux-kernel - Re: [PATCH 2/6] perf: Support branch events logging

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Message-ID: <20230414192407.GA778423@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Apr 2023 21:24:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, irogers@...gle.com, adrian.hunter@...el.com,
        eranian@...gle.com
Subject: Re: [PATCH 2/6] perf: Support branch events logging

On Fri, Apr 14, 2023 at 01:53:24PM -0400, Liang, Kan wrote:
> 
> 
> On 2023-04-14 12:09 p.m., Peter Zijlstra wrote:
> > On Fri, Apr 14, 2023 at 11:56:41AM -0400, Liang, Kan wrote:
> >>> If it were to only support 4, then
> >>> we're in counter scheduling contraint hell again 
> >>
> >> Unfortunately, yes.
> >>
> >>> and we need to somehow
> >>> group all these things together with the LBR event.
> >>
> >> Group will bring many limits for the usage. For example, I was told
> >> there could be someone wants to use it with multiplexing.
> > 
> > You can create two groups, each with an LBR event, no?
> 
> If we put everything in a group, that will make the enabling much
> simpler. I don't think the perf tool needs the order information
> anymore. Because the kernel enables the events one by one in a group.
> The kernel just need to convert the information from the counter order
> to the enabling order and dump to user space.

I never understood the whole order thing. What was it trying to do?

> But if we have two groups with LBR event, the order information is still
> required. Why we still want to group things?

Why would you need that; what is that whole order nonsense about?

{e1, e2, e3, e4}, {e5, e6, e7, e8} with e1 and e5 both having LBR on
just works no?

Since they have LBR and that extra sample flag they all get a 0-3
constraint.

Since both e1 and e5 use LBR, they're mutually exclusive, either e1 or
e5 group runs.