Re: [PATCH] sched_ext: Check bpf_timer_start return values in scx_qmap

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Medium] central_timerfn() unconditionally re-arms the timer with BPF_F_TIMER_CPU_PIN and ignores the return value, causing silent failure on older kernels.
--

commit b5a91ea2da5adc3305be9b35666d557645b76fd2
Author: Wanwu Li <liwanwu@kylinos.cn>

sched_ext: Check bpf_timer_start return values in scx_qmap

This commit adds return value checks to bpf_timer_start() calls in several
timer functions within scx_qmap. If the timer fails to re-arm, it now raises
an error via scx_bpf_error() rather than silently failing.

> diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
> index 5bb8b90a275a3..9f6e61d7ca071 100644
> --- a/tools/sched_ext/scx_qmap.bpf.c
> +++ b/tools/sched_ext/scx_qmap.bpf.c
> @@ -1246,7 +1246,8 @@ static int monitor_timerfn(void *map, int *key, struct bpf_timer *timer)

[ ... ]

> -	bpf_timer_start(timer, ONE_SEC_IN_NS, 0);
> +	if (bpf_timer_start(timer, ONE_SEC_IN_NS, 0))
> +		scx_bpf_error("failed to re-arm stats timer");
>  	return 0;
>  }

[Severity: Medium]
This is a pre-existing issue, but while adding these return value checks here,
should we apply the same fix to central_timerfn() in scx_central.bpf.c?

It unconditionally re-arms the timer with BPF_F_TIMER_CPU_PIN and ignores
the return value:

tools/sched_ext/scx_central.bpf.c:central_timerfn() {
    ...
    bpf_timer_start(timer, TIMER_INTERVAL_NS, BPF_F_TIMER_CPU_PIN);
    __sync_fetch_and_add(&nr_timers, 1);
    return 0;
}

On older kernels that don't support BPF_F_TIMER_CPU_PIN, this fails with
-EINVAL. While start_central_timer() gracefully falls back to flags = 0
if unsupported, the timer callback unconditionally passes the flag. Because
the return value is ignored, the timer permanently stops, potentially
starving tasks.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260827080738.829103-3-liwanwu@kylinos.cn?part=1