linux-kernel - Re: [RFC v2] perf: Rewrite core context handling

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Message-ID: <YwXSyWE9BqTwi5aL@worktop.programming.kicks-ass.net>
Date:   Wed, 24 Aug 2022 09:27:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, songliubraving@...com,
        eranian@...gle.com, alexey.budankov@...ux.intel.com,
        ak@...ux.intel.com, mark.rutland@....com, megha.dey@...el.com,
        frederic@...nel.org, maddy@...ux.ibm.com, irogers@...gle.com,
        kim.phillips@....com, linux-kernel@...r.kernel.org,
        santosh.shukla@....com
Subject: Re: [RFC v2] perf: Rewrite core context handling

On Wed, Aug 24, 2022 at 10:37:36AM +0530, Ravi Bangoria wrote:

> > Now, I suppose making that:
> > 
> >   {-1, NULL, NULL}, {cpu, NULL, NULL}
> > 
> > could work, but wouldn't iterating the the tree be more expensive than
> > just finding the sub-trees as we do now?
> 
> pmu=NULL can be used while scheduling entire context. We can just traverse
> through all pmu events of both cpu subtrees.

But imagine the case where we have 50 event for a PMU that can only
schedule 8. Then we have to iterate 42 events for naught instead of
directly jumping to the next PMU.