linux-kernel - Re: [PATCH 1/2] sched_ext: Add an event, SCX_EV_ENQ_SLICE

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Message-ID: <612ce321-893a-468c-bbda-0220edab5036@igalia.com>
Date: Fri, 7 Feb 2025 15:28:01 +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 1/2] sched_ext: Add an event, SCX_EV_ENQ_SLICE_DFL

Hi Andrea,

On 25. 2. 7. 15:17, Andrea Righi wrote:
> Hi Changwoo,
> 
> On Fri, Feb 07, 2025 at 12:13:37PM +0900, Changwoo Min wrote:
>> Add a core event, SCX_EV_ENQ_SLICE_DFL, which represents how many
>> tasks have been enqueued (or pick_task-ed) with a default time slice
>> (SCX_SLICE_DFL).
>>
>> 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.
>>
>> __scx_add_event() is used since the caller holds an rq lock,
>> so the preemption has already been disabled.
> 
> We may want to consider select_task_rq_scx() as well, when ops.select_cpu()
> is not implemented (or during rq_bypass).
> 
> In this case, if scx_select_cpu_dfl() finds an idle CPU, we implicitly
> dispatch the task to the local DSQ with SCX_SLICE_DFL.

You are right. I will add it too.

Thanks!
-- Changwoo