linux-kernel - Re: [PATCH 5/7] perf: Optimise topology iteration

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Date:	Tue, 04 Jan 2011 15:13:02 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Stephane Eranian <eranian@...gle.com>,
	"robert.richter@....com" <robert.richter@....com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/7] perf: Optimise topology iteration

On Mon, 2011-01-03 at 23:20 +0800, Andi Kleen wrote:
> On Mon, Jan 03, 2011 at 12:02:10PM +0100, Peter Zijlstra wrote:
> > On Mon, 2010-12-27 at 23:38 +0800, Lin Ming wrote:
> > > Currently we iterate the full machine looking for a matching core_id/nb
> > > for the percore and the amd northbridge stuff , using a smaller topology
> > > mask makes sense. 
> > 
> > Does topology_thread_cpumask() include offline cpus? I tried looking at
> > it, but I cannot find any code clearing bits in that mask on offline.
> 
> The problem is not only at offline, but also at online between CPUs
> going online.  I don't think the patch is a good idea and it doesn't

I didn't see the problem.

Assume logical cpu 3, 7 are 2 threads in a core, and they are
plug/unpluged as below sequence,
CPU 3 offline, CPU7 offline, CPU3 online, CPU7 online

1. After cpu3 offline

topology_thread_cpumask(3) returns empty
topology_thread_cpumask(7) returns 7

2. After CPU7 offline

topology_thread_cpumask(3) returns empty
topology_thread_cpumask(7) returns empty

3. When CPU3 online, calling intel_pmu_cpu_starting

topology_thread_cpumask(3) returns 3
topology_thread_cpumask(7) returns empty

        for_each_cpu(i, topology_thread_cpumask(cpu)) {
                struct intel_percore *pc = per_cpu(cpu_hw_events, i).per_core;

                if (pc && pc->core_id == core_id) {
                        kfree(cpuc->per_core);
                        cpuc->per_core = pc;
                        break;
                }
        }

Above "if" statement will not be executed, because pc->core_id was
initialized to -1 in intel_pmu_cpu_prepare.

4. When CPU7 online, calling intel_pmu_cpu_starting

topology_thread_cpumask(3) returns 3, 7
topology_thread_cpumask(7) returns 3, 7


        for_each_cpu(i, topology_thread_cpumask(cpu)) {
                struct intel_percore *pc = per_cpu(cpu_hw_events, i).per_core;

                if (pc && pc->core_id == core_id) {
                        kfree(cpuc->per_core);
                        cpuc->per_core = pc;
                        break;
                }
        }

        cpuc->per_core->core_id = core_id;
        cpuc->per_core->refcnt++;

Above "if" statement will be executed and the per_core data allocated
for cpu7 will be freed.

All above is right, or could you explain more about the problem at CPUs
offline and online?

Thanks,
Lin Ming

> even have any advantages either since this is a initialization only
> slow path.
> 
> -Andi


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