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Date: Tue, 27 Sep 2022 20:33:23 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Peter Zijlstra <peterz@...radead.org>, <mingo@...hat.com>,
<will@...nel.org>, Waiman Long <longman@...hat.com>
CC: <linux-kernel@...r.kernel.org>, <"<boqun.feng"@gmail.com>
Subject: Re: locking/rwsem: RT throttling issue due to RT task hogging the cpu
I was thinking if below patch can also help on this issue.
--------------------------------->O---------------------------
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 65f0262..dbe3e16 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -628,8 +628,8 @@ static inline bool rwsem_try_write_lock(struct
rw_semaphore *sem,
new = count;
if (count & RWSEM_LOCK_MASK) {
- if (has_handoff || (!rt_task(waiter->task) &&
- !time_after(jiffies,
waiter->timeout)))
+ if (has_handoff || (rt_task(waiter->task) &&
waiter != first) ||
+ (!rt_task(waiter->task) &&
!time_after(jiffies, waiter->timeout)))
return false;
-Mukesh
On 9/26/2022 5:16 PM, Mukesh Ojha wrote:
> Hi,
>
> Any comments on this issue would be helpful.
>
> Thanks,
> Mukesh
>
> On 9/20/2022 9:49 PM, Mukesh Ojha wrote:
>> Hi,
>>
>> We are observing one issue where, sem->owner is not set and
>> sem->count=6 [1] which means both RWSEM_FLAG_WAITERS and
>> RWSEM_FLAG_HANDOFF bits are set. And if unfold the sem->wait_list we
>> see the following order of process waiting [2] where [a] is waiting
>> for write, while [b],[c] are waiting for read and [d] is the RT task
>> for which waiter.handoff_set=true and it is continuously running on
>> cpu7 and not letting the first write waiter [a] on cpu7.
>>
>> [1]
>>
>> sem = 0xFFFFFFD57DDC6680 -> (
>> count = (counter = 6),
>> owner = (counter = 0),
>>
>> [2]
>>
>> [a] kworker/7:0 pid: 32516 ==> [b] iptables-restor pid: 18625 ==>
>> [c]HwBinder:1544_3 pid: 2024 ==> [d] RenderEngine pid: 2032 cpu: 7
>> prio:97 (RT task)
>>
>>
>> Sometime back, Waiman has suggested this which could help in RT task
>> leaving the cpu.
>>
>> https://lore.kernel.org/all/8c33f989-8870-08c6-db12-521de634b34e@redhat.com/
>>
>>
>> --------------------------------->O----------------------------
>>
>> From c6493edd7a5e4f597ea55ff0eb3f1d763b335dfc Mon Sep 17 00:00:00 2001
>> 2 From: Waiman Long <longman@...hat.com>
>> 3 Date: Tue, 20 Sep 2022 20:50:45 +0530
>> 4 Subject: [PATCH] locking/rwsem: Yield the cpu after doing handoff
>> optimistic
>> 5 spinning
>> 6
>> 7 It is possible the new lock owner (writer) can be preempted
>> before setting
>> 8 the owner field and if the current(e.g RT task) waiter is the
>> task that
>> 9 preempts the new lock owner, it will hand_off spin loop for a
>> long time.
>> 10 Avoid wasting cpu time and delaying the release of the lock by
>> yielding
>> 11 the cpu if handoff optimistic spinning has been done multiple
>> times with
>> 12 NULL owner.
>> 13
>> 14 Signed-off-by: Waiman Long <longman@...hat.com>
>> 15 Signed-off-by: Mukesh Ojha <quic_mojha@...cinc.com>
>> 16 ---
>> 17 kernel/locking/rwsem.c | 15 ++++++++++++++-
>> 18 1 file changed, 14 insertions(+), 1 deletion(-)
>> 19
>> 20 diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
>> 21 index 65f0262..a875758 100644
>> 22 --- a/kernel/locking/rwsem.c
>> 23 +++ b/kernel/locking/rwsem.c
>> 24 @@ -361,6 +361,8 @@ enum rwsem_wake_type {
>> 25 */
>> 26 #define MAX_READERS_WAKEUP 0x100
>> 27
>> 28 +#define MAX_HANDOFF_SPIN 10
>> 29 +
>> 30 static inline void
>> 31 rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter
>> *waiter)
>> 32 {
>> 33 @@ -1109,6 +1111,7 @@ rwsem_down_write_slowpath(struct
>> rw_semaphore *sem, int state)
>> 34 {
>> 35 struct rwsem_waiter waiter;
>> 36 DEFINE_WAKE_Q(wake_q);
>> 37 + int handoff_spins = 0;
>> 38
>> 39 /* do optimistic spinning and steal lock if possible */
>> 40 if (rwsem_can_spin_on_owner(sem) &&
>> rwsem_optimistic_spin(sem)) {
>> 41 @@ -1167,6 +1170,14 @@ rwsem_down_write_slowpath(struct
>> rw_semaphore *sem, int state)
>> 42 * has just released the lock, OWNER_NULL will be
>> returned.
>> 43 * In this case, we attempt to acquire the lock
>> again
>> 44 * without sleeping.
>> 45 + *
>> 46 + * It is possible the new lock owner (writer) can
>> be preempted
>> 47 + * before setting the owner field and if the
>> current(e.g RT task)
>> 48 + * waiter is the task that preempts the new lock
>> owner, it will
>> 49 + * spin in this loop for a long time. Avoid
>> wasting cpu time
>> 50 + * and delaying the release of the lock by
>> yielding the cpu if
>> 51 + * handoff optimistic spinning has been done
>> multiple times with
>> 52 + * NULL owner.
>> 53 */
>> 54 if (waiter.handoff_set) {
>> 55 enum owner_state owner_state;
>> 56 @@ -1175,8 +1186,10 @@ rwsem_down_write_slowpath(struct
>> rw_semaphore *sem, int state)
>> 57 owner_state = rwsem_spin_on_owner(sem);
>> 58 preempt_enable();
>> 59
>> 60 - if (owner_state == OWNER_NULL)
>> 61 + if ((owner_state == OWNER_NULL) &&
>> (handoff_spins < MAX_HANDOFF_SPIN)) {
>> 62 + handoff_spins++;
>> 63 goto trylock_again;
>> 64 + }
>> 65 }
>> 66
>> 67 schedule();
>> 68 --
>> 69 2.7.4
>> 70
>>
>>
>> -Mukesh
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