linux-kernel - Re: [PATCH RFC bpf-next v2 02/10] bpf/helpers: introduce sleepable timers

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Date: Tue, 20 Feb 2024 18:49:44 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Benjamin Tissoires <bentiss@...nel.org>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-input@...r.kernel.org, linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Jiri Kosina <jikos@...nel.org>, Benjamin Tissoires <benjamin.tissoires@...hat.com>, 
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH RFC bpf-next v2 02/10] bpf/helpers: introduce sleepable
 timers

On Fri, Feb 16, 2024 at 03:58:20PM +0100, Benjamin Tissoires wrote:
> 
> So (and this also answers your second email today) I'm thinking at:
> - have multiple flags to control the timer (with dedicated timer_cb
>   kernel functions):
>   - BPF_F_TIMER_HRTIMER (default)
>   - BPF_F_TIMER_WORKER (no timer, just workqueue)

These two make sense, but

>   - BPF_F_TIMER_DELAYED_WORKER (hrtimer + workqueue, or actual
>     delayed_work, but that's re-implementing stuffs)

This one doesn't.
Unlike hrtimer the workqueue is non deterministic.
Requesting a callback after a specific delay only to be randomized
by the workqueue is a confusing UX to give to bpf progs.
If bpf author really want to do something like that they can implement
such anti-feature manually with two bpf_timers.

Later we'll add a selector for WQ. At that time we'd need to decide
whether to use a dedicated kthread or any of system_*_wq or WQ_BH.
For now I'd only expose 'sleepable' as a guarantee in bpf api.
Hence BPF_F_TIMER_SLEEPABLE is the only extra bit in flags for bpf_timer_start().
Not sure whether it's needed in bpf_timer_init() too.