linux-kernel - Re: [PATCH v8] perf: add qcom l2 cache perf events driver

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Message-ID: <09ac83e5-4cab-0a1e-b9bd-93bf366aa82d@codeaurora.org>
Date:   Wed, 1 Feb 2017 14:57:55 -0500
From:   "Leeder, Neil" <nleeder@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Mark Langsdorf <mlangsdo@...hat.com>,
        Mark Salter <msalter@...hat.com>, Jon Masters <jcm@...hat.com>,
        Timur Tabi <timur@...eaurora.org>, cov@...eaurora.org,
        nleeder@...eaurora.org
Subject: Re: [PATCH v8] perf: add qcom l2 cache perf events driver

Mark,
Thanks for all the comments and code samples. I will update the patch
and repost.

On 1/30/2017 10:19 AM, Mark Rutland wrote:
> On Mon, Jan 16, 2017 at 01:52:47PM -0500, Neil Leeder wrote:

> This is fine as is, but just for my understanding, I take it that the
> locking is only strictly required to be per-cluster?

Correct, only per-cluster.

>> +		conflict_event =
>> +			cluster->events[cluster->group_to_counter[group]];
>> +		if (conflict_event != event) {
>
> If it's possible for conflict_event == event, it sounds like this is
> fragile to the order events are added or removed.
>
> When does conflict_event == event?

Hmmm, when testing this many months ago I saw filter_match being called
multiple times with the same event so it would conflict with itself. I
don't see this now so if I can't reproduce it I'll remove the test.

>> +			l2cache_pmu = to_l2cache_pmu(event->pmu);
>> +			chwc = &conflict_event->hw;
>> +			dev_dbg_ratelimited(&l2cache_pmu->pdev->dev,
>> +				    "column exclusion between events %lx %lx\n",
>> +				    hwc->config_base, chwc->config_base);
>
> This could happen fairly often, and even with ratelimiting we're doing
> unnecessary work. Can we get rid of the debug logic here?

Does this happen often? We're not doing sampling or running in task
mode which involve rapid adds and deletes of events, so this really
only happens when enabling an event. I'd like to keep some type of
message because it's the only thing that will inform a user why the
event they specified didn't count anything. And really why I'd prefer
it to be a warn instead of dbg, but I can live with the dbg.

> To ensure that we reliably reset clusters, and so as to avoid redundant
> cpumask copies, I think we should rewrite the hotplug callbacks
> something like as follows.

I'm fine with this except for one case:

> 	/* Try to find a new owner */
> 	target = cpumask_any_any(cpu_online_mask, &cluster->cluster_cpus);

(assuming typo for cpumask_any_and)
The current cpu is still in cpu_online_mask, so we can end up with
target==cpu. So we need the AND, and then an any_but(..., cpu), which
is why I had that originally.

>> +	cluster->l2cache_pmu = l2cache_pmu;
>> +	for_each_present_cpu(cpu) {
>> +		if (topology_physical_package_id(cpu) == fw_cluster_id) {
>> +			cpumask_set_cpu(cpu, &cluster->cluster_cpus);
>> +			per_cpu(pmu_cluster, cpu) = cluster;
>> +		}
>> +	}
>
> Sorry to go back-and-forth on this particular point, but I am worried
> that this is a little fragile, since the topology stuff is largely a
> guess, and I can imagine it might change in future.
>
> What exactly is the fw_cluster_id? Is it always a particular Aff<N>
> field? Might it differ in future?

fw_cluster_id is the cluster id assigned by firmware, which matches
Aff1 for CPUs without multi-threading, Aff2 for those with. I think
last time we discussed this I agreed to add a comment block regarding
the expectation that this stays the same, which I didn't do. I can add
it now.

I understand your concern about future changes, but I believe this will
remain as-is for the current generation of Qualcomm Technologies
processors, and although I can't talk about unannounced devices I
believe that this item will not be a problem for this driver in future.
And if that somehow turns out not to be the case, updating this to
whatever new scheme is used should be fairly self-contained.

Thanks,

Neil
-- 
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.