linux-kernel - Re: [PATCH sched_ext/for-6.11 2/2] sched_ext: Implement scx_bpf_consume

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Date: Fri, 28 Jun 2024 13:12:59 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
	David Vernet <void@...ifault.com>
Subject: Re: [PATCH sched_ext/for-6.11 2/2] sched_ext: Implement
 scx_bpf_consume_task()

Hello, again.

On Fri, Jun 28, 2024 at 01:04:04PM -1000, Tejun Heo wrote:
...
> Not a stupid question at all. It's just that all the existing interface is
> based on IDs. This is partly because there's not much the BPF code can do
> with the DSQ data structure and partly because DSQs are usually not accessed
> multiple times in sequence (ie. if the BPF code isn't going to look it up
> and hold it persistently, it's going to have to look it up each time
> anyway).
> 
> The multiple lookups aren't the end of the world. They're all on a resizing
> hashtable, so lookups should be pretty low cost. It's just a little bit sad
> to look at.

Just a bit of addition and a question. scx_bpf_consume_task() is maybe named
too generically and I have a hard time imagining it being useful outside
iteration loop. So, it does work out kinda neatly if we can tie the whole
thing (DSQ lookup, barrier seq) to the iterator.

The reason why this becomes nasty is because I can't pass the pointer to the
iterator to a kfunc, so maybe allowing that can be a solution here too?

Thanks.

-- 
tejun