Messages in this thread | | | Date | Tue, 6 Sep 2022 18:13:14 +0530 | Subject | Re: [PATCH] locking/rwsem: Disable preemption while trying for rwsem lock | From | Mukesh Ojha <> |
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Hi,
On 9/3/2022 2:25 AM, Waiman Long wrote: > > On 9/1/22 06:28, Mukesh Ojha wrote: >> From: Gokul krishna Krishnakumar <quic_gokukris@quicinc.com> >> >> Make the region inside the rwsem_write_trylock non preemptible. >> >> We observe RT task is hogging CPU when trying to acquire rwsem lock >> which was acquired by a kworker task but before the rwsem owner was set. >> >> Here is the scenario: >> 1. CFS task (affined to a particular CPU) takes rwsem lock. >> >> 2. CFS task gets preempted by a RT task before setting owner. >> >> 3. RT task (FIFO) is trying to acquire the lock, but spinning until >> RT throttling happens for the lock as the lock was taken by CFS task. > > Note that the spinning is likely caused by the following code in > rwsem_down_write_slowpath(): > > 1163 /* > 1164 * After setting the handoff bit and failing to > acquire > 1165 * the lock, attempt to spin on owner to accelerate > lock > 1166 * transfer. If the previous owner is a on-cpu > writer and it > 1167 * has just released the lock, OWNER_NULL will be > returned. > 1168 * In this case, we attempt to acquire the lock again > 1169 * without sleeping. > 1170 */ > 1171 if (waiter.handoff_set) { > 1172 enum owner_state owner_state; > 1173 > 1174 preempt_disable(); > 1175 owner_state = rwsem_spin_on_owner(sem); > 1176 preempt_enable(); > 1177 > 1178 if (owner_state == OWNER_NULL) > 1179 goto trylock_again; > 1180 } > > rwsem_optimistic_spin() limits RT task one additional attempt if > OWNER_NULL is returned. There is no such limitation in this loop. So an > alternative will be to put a limit on the number of times an OWNER_NULL > return values will be allowed to continue spinning without sleeping. > That put the burden on the slowpath instead of in the fastpath. > > Other than the slight overhead in the fastpath, the patch should work too. > > Acked-by: Waiman Long <longman@redhat.com>
Thanks Waiman for your time and suggestion. Would like to take others opinion as well.
-Mukesh
> > Cheers, > Longman >
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