Re: [RFC PATCH] xprtrdma: Move long delayed work on system_dfl_long_wq

[Chuck Lever]

On Thu, Apr 30, 2026, at 4:54 AM, Marco Crivellari wrote:
> Currently the code enqueue work items using {queue|mod}_delayed_work(),
> using system_long_wq. This workqueue should be used when long works are
> expected, but it is a per-cpu workqueue.
>
> This is important because queue_delayed_work() queue the work using:
>
>    queue_delayed_work_on(WORK_CPU_UNBOUND, ...);
>
> Note that WORK_CPU_UNBOUND = NR_CPUS.
>
> This would end up calling __queue_delayed_work() that does:
>
>     if (housekeeping_enabled(HK_TYPE_TIMER)) {
>     //      [....]
>     } else {
>             if (likely(cpu == WORK_CPU_UNBOUND))
>                     add_timer_global(timer);
>             else
>                     add_timer_on(timer, cpu);
>     }
>
> So when cpu == WORK_CPU_UNBOUND the timer is global and is
> not using a specific CPU. Later, when __queue_work() is called:
>
>     if (req_cpu == WORK_CPU_UNBOUND) {
>             if (wq->flags & WQ_UNBOUND)
>                     cpu = wq_select_unbound_cpu(raw_smp_processor_id());
>             else
>                     cpu = raw_smp_processor_id();
>     }
>
> Because the wq is not unbound, it takes the CPU where the timer
> fired and enqueue the work on that CPU.
> The consequence of all of this is that the work can run anywhere,
> depending on where the timer fired.
>
> Recently, a new unbound workqueue specific for long running work has
> been added:
>
>    c116737e972e ("workqueue: Add system_dfl_long_wq for long unbound works")
>
> So change system_long_wq with system_dfl_long_wq so that the work may
> benefit from scheduler task placement.

The patch description confuses me.

The message ends with "the work can run anywhere, depending on where
the timer fired." Read literally, "can run anywhere" sounds like a
feature, not a bug — and the proposed fix (WQ_UNBOUND) also lets it
run anywhere, just via a different selection path. Without a sentence
saying "and that anywhere includes isolated CPUs, which we don't want,"
the reader is left to fill in the gap.                                

So, could the commit message lead with the motivation? My guess is that
this is about respecting HK_TYPE_TIMER housekeeping on isolated systems,
which system_long_wq cannot do because its per-CPU pool ignores the
housekeeping mask once the global timer fires. If that is the case,
please say so directly and the mechanism trace becomes a supporting
argument rather than the whole argument.


-- 
Chuck Lever