linux-kernel - Re: [PATCH 0/2] sched_ext: Add a core event and update scx schedulers

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Message-ID: <7623f8e9-d598-4c0f-8f65-5cb0cf563d4e@igalia.com>
Date: Fri, 7 Feb 2025 15:49:24 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Andrea Righi <arighi@...dia.com>
Cc: tj@...nel.org, void@...ifault.com, kernel-dev@...lia.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] sched_ext: Add a core event and update scx schedulers

Hello,

On 25. 2. 7. 15:24, Andrea Righi wrote:
> Hi,
> 
> On Fri, Feb 07, 2025 at 12:13:36PM +0900, Changwoo Min wrote:
>> This patchset introduces a new event, SCX_EV_ENQ_SLICE_DFL, and updates
>> two scx schedulers -- scx_qmap and scx_central -- to print out the new
>> event.
>>
>> SCX_EV_ENQ_SLICE_DFL counts how many times the tasks' time slice is set
>> to the default value (SCX_SLICE_DFL) by the sched_ext core in the enqueue
>> and pick_next paths.
>>
>> Scheduling a task with SCX_SLICE_DFL unintentionally would be a source
>> of latency spikes because SCX_SLICE_DFL is relatively long (20 msec).
>> Thus, soaring the SCX_EV_ENQ_SLICE_DFL value would be a sign of BPF
>> scheduler bugs, causing latency spikes.
> 
> Not directly related to this patch set, but as a general thought: would it
> be useful to introduce ops->slice_ms (in sched_ext_ops) to override
> SCX_SLICE_DFL?
> 
> With that, schedulers that care about latency could set a smaller default
> time slice to prevent potential spikes caused by the implicit use of
> SCX_SLICE_DFL.
> 
> Opinions?

That sounds like a good idea!

Regards,
Changwoo Min