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Message-ID: <ab67aa4d-bc58-4c80-a95b-6bf21fb5417f@linux.intel.com>
Date: Wed, 12 Jun 2024 15:25:00 -0400
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Subject: Re: [PATCH 1/8] perf/x86/uncore: Save the unit control address of all
 units
To: Tim Chen <tim.c.chen@linux.intel.com>, peterz@infradead.org,
 mingo@kernel.org, linux-kernel@vger.kernel.org
Cc: acme@kernel.org, namhyung@kernel.org, irogers@google.com,
 eranian@google.com, ak@linux.intel.com, yunying.sun@intel.com
References: <20240610201619.884021-1-kan.liang@linux.intel.com>
 <20240610201619.884021-2-kan.liang@linux.intel.com>
 <0f1ba5d8ecc62f774590077b2f88f5b64dd98452.camel@linux.intel.com>
 <eb5d91d1-2898-45e0-a2d3-aa5c66155911@linux.intel.com>
 <47fad008a152957c87e9846fdbf40746b6e2871a.camel@linux.intel.com>
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From: "Liang, Kan" <kan.liang@linux.intel.com>
In-Reply-To: <47fad008a152957c87e9846fdbf40746b6e2871a.camel@linux.intel.com>
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On 2024-06-12 1:33 p.m., Tim Chen wrote:
> On Wed, 2024-06-12 at 10:49 -0400, Liang, Kan wrote:
>>
>>
>> The id and pmu_idx+die can all be used as a key to search the RB tree in
>> different places.
>>
>> The id is the physical ID of a unit. The search via id is invoked when
>> adding a new unit. Perf needs to make sure that the same PMU idx
>> (logical id) is assigned to the unit with the same physical ID. Because
>> the units with the same physical ID in different dies share the same PMU.
> 
> This info about having same physical ID implies the same PMU
> is worth mentioning in a comment and will be quite helpful in
> understanding the rb-tree organization.
>

Sure I will update the description to explain the choice. Thanks.

Thanks,
Kan

> Thanks.
> 
> Tim
>>
>> The pmu_idx+die key is used when setting the cpumask. Please see
>> intel_uncore_find_discovery_unit_id() in the patch 2. Perf wants to
>> understand on which dies the given PMU is available.
>>
>> Since different keys can be used to search the RB tree, I think one of
>> them has to traverse the whole tree. At the stage of adding a new unit,
>> the tree is not complete yet. It minimizes the impact of the O(N)
>> search. So I choose the pmu_idx+die rather than id.
>>
>> Also, the driver only does once to build the tree and set the cpumask at
>> driver load time. I think the O(N) should be acceptable here.
>>
>> Thanks,
>> Kan
>