[RFC PATCH] ata: libata-scsi: Move long delayed work on system_dfl_long_wq

[Marco Crivellari]

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.

Signed-off-by: Marco Crivellari <marco.crivellari@xxxxxxxx>
---
 drivers/ata/libata-scsi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index f44612e269a4..6733f2b14521 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -4753,7 +4753,7 @@ void ata_scsi_scan_host(struct ata_port *ap, int sync)
 			     "WARNING: synchronous SCSI scan failed without making any progress, switching to async\n");
 	}
 
-	queue_delayed_work(system_long_wq, &ap->hotplug_task,
+	queue_delayed_work(system_dfl_long_wq, &ap->hotplug_task,
 			   round_jiffies_relative(HZ));
 }
 
-- 
2.53.0