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Re: [PATCH] xen/credit: Uncount units dropped by csched_schedule()
- To: Lin Liu <lin.liu01@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Mon, 9 Nov 2026 07:43:34 +0100
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- Cc: Bengang Yuan <bengang.yuan@xxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, George Dunlap <gwd@xxxxxxxxxxxxxx>
- Delivery-date: Mon, 09 Nov 2026 06:43:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.11.26 11:09, Lin Liu wrote:
csched_schedule() takes snext off the runq without decrementing
nr_runnable, as it is about to run. A unit that stops being runnable
is normally removed and uncounted by csched_unit_sleep(), but if the
pick loop drops it first, the sleep finds nothing to do and the unit
stays counted:
CPU A CPU B
domain_pause() csched_schedule(), lock held
atomic_inc(&d->pause_count)
__runq_remove(snext)
vcpu_sleep_sync() !unit_runnable_state(): drop
waits for lock unlock
csched_unit_sleep()
not running, not queued:
nothing to do
domain_unpause()
vcpu_wake()
runq_insert() nr_runnable +1 again
Decrement nr_runnable when dropping the unit.
Fixes: 7c7b407e7772 ("xen/sched: introduce unit_runnable_state()")
Reported-by: Bengang Yuan <bengang.yuan@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Lin Liu <lin.liu01@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Juergen
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