Re: [RFC PATCH 0/3] blk-mq: rework queue freeze and preempt-only
From: Bart Van Assche
Date: Thu Sep 20 2018 - 13:07:16 EST
On Thu, 2018-09-20 at 18:18 +-0800, Jianchao Wang wrote:
+AD4 The current queue freeze depending on percpu+AF8-ref+AF8-kil/reinit has a limit that
+AD4 we have drain the requests before unfreeze the queue.
+AD4
+AD4 Let's rework the queue freeze feature as following:
+AD4 1. introduce +AF8AXw-percpu+AF8-ref+AF8-get+AF8-many.
+AD4 It is same with original percpu+AF8-ref+AF8-get+AF8-many, but just need callers to provide
+AD4 sched rcu critical section. We will put the +AF8AXw-percpu+AF8-ref+AF8-get+AF8-many and our own
+AD4 condition checking under rcu+AF8-read+AF8-lock+AF8-sched. With this new helper interface,
+AD4 we could save an extra rcu+AF8-read+AF8-lock+AF8-sched.
+AD4 2. rework the blk+AF8-queue+AF8-enter as:
+AD4
+AD4 rcu+AF8-read+AF8-lock+AF8-sched()
+AD4 if condition check true
+AD4 +AF8AXw-percpu+AF8-ref+AF8-get+AF8-many(+ACY-q-+AD4-q+AF8-usage+AF8-counter, 1)
+AD4 else
+AD4 goto wait
+AD4 rcu+AF8-read+AF8-unlock+AF8-sched()
+AD4 3. use percpu+AF8-ref+AF8-switch+AF8-to+AF8-atomic/percpu to switch mode directly.
+AD4
+AD4 Then we could unfreeze the queue w/o draining requests.
+AD4 In addition, preempt-only mode code could be simplified.
Hello Jianchao,
+AD0
Thanks for having taken a look. However, the approach of this patch series
may be more complicated than necessary. Had you considered something like
the patch below?
Thanks,
Bart.
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 20fdd78b75c7..d384ab700afd 100644
--- a/block/blk-mq.c
+-+-+- b/block/blk-mq.c
+AEAAQA -199,7 +-199,7 +AEAAQA void blk+AF8-mq+AF8-unfreeze+AF8-queue(struct request+AF8-queue +ACo-q)
freeze+AF8-depth +AD0 atomic+AF8-dec+AF8-return(+ACY-q-+AD4-mq+AF8-freeze+AF8-depth)+ADs
WARN+AF8-ON+AF8-ONCE(freeze+AF8-depth +ADw 0)+ADs
if (+ACE-freeze+AF8-depth) +AHs
- percpu+AF8-ref+AF8-reinit(+ACY-q-+AD4-q+AF8-usage+AF8-counter)+ADs
+- percpu+AF8-ref+AF8-resurrect(+ACY-q-+AD4-q+AF8-usage+AF8-counter)+ADs
wake+AF8-up+AF8-all(+ACY-q-+AD4-mq+AF8-freeze+AF8-wq)+ADs
+AH0
+AH0
diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 009cdf3d65b6..b297cd1cd4f1 100644
--- a/include/linux/percpu-refcount.h
+-+-+- b/include/linux/percpu-refcount.h
+AEAAQA -108,6 +-108,7 +AEAAQA void percpu+AF8-ref+AF8-switch+AF8-to+AF8-atomic+AF8-sync(struct percpu+AF8-ref +ACo-ref)+ADs
void percpu+AF8-ref+AF8-switch+AF8-to+AF8-percpu(struct percpu+AF8-ref +ACo-ref)+ADs
void percpu+AF8-ref+AF8-kill+AF8-and+AF8-confirm(struct percpu+AF8-ref +ACo-ref,
percpu+AF8-ref+AF8-func+AF8-t +ACo-confirm+AF8-kill)+ADs
+-void percpu+AF8-ref+AF8-resurrect(struct percpu+AF8-ref +ACo-ref)+ADs
void percpu+AF8-ref+AF8-reinit(struct percpu+AF8-ref +ACo-ref)+ADs
/+ACoAKg
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 9f96fa7bc000..054e7650cd35 100644
--- a/lib/percpu-refcount.c
+-+-+- b/lib/percpu-refcount.c
+AEAAQA -356,11 +-356,31 +AEAAQA EXPORT+AF8-SYMBOL+AF8-GPL(percpu+AF8-ref+AF8-kill+AF8-and+AF8-confirm)+ADs
+ACo-/
void percpu+AF8-ref+AF8-reinit(struct percpu+AF8-ref +ACo-ref)
+AHs
+- WARN+AF8-ON+AF8-ONCE(+ACE-percpu+AF8-ref+AF8-is+AF8-zero(ref))+ADs
+-
+- percpu+AF8-ref+AF8-resurrect(ref)+ADs
+-+AH0
+-EXPORT+AF8-SYMBOL+AF8-GPL(percpu+AF8-ref+AF8-reinit)+ADs
+-
+-/+ACoAKg
+- +ACo percpu+AF8-ref+AF8-resurrect - modify a percpu refcount from dead to live
+- +ACo +AEA-ref: perpcu+AF8-ref to resurrect
+- +ACo
+- +ACo Modify +AEA-ref so that it's in the same state as before percpu+AF8-ref+AF8-kill() was
+- +ACo called. +AEA-ref must have be dead but not exited.
+- +ACo
+- +ACo Note that percpu+AF8-ref+AF8-tryget+AFsAXw-live+AF0() are safe to perform on +AEA-ref while
+- +ACo this function is in progress.
+- +ACo-/
+-void percpu+AF8-ref+AF8-resurrect(struct percpu+AF8-ref +ACo-ref)
+-+AHs
+- unsigned long +AF8AXw-percpu +ACo-percpu+AF8-count+ADs
unsigned long flags+ADs
spin+AF8-lock+AF8-irqsave(+ACY-percpu+AF8-ref+AF8-switch+AF8-lock, flags)+ADs
- WARN+AF8-ON+AF8-ONCE(+ACE-percpu+AF8-ref+AF8-is+AF8-zero(ref))+ADs
+- WARN+AF8-ON+AF8-ONCE(+ACE(ref-+AD4-percpu+AF8-count+AF8-ptr +ACY +AF8AXw-PERCPU+AF8-REF+AF8-DEAD))+ADs
+- WARN+AF8-ON+AF8-ONCE(+AF8AXw-ref+AF8-is+AF8-percpu(ref, +ACY-percpu+AF8-count))+ADs
ref-+AD4-percpu+AF8-count+AF8-ptr +ACYAPQ +AH4AXwBf-PERCPU+AF8-REF+AF8-DEAD+ADs
percpu+AF8-ref+AF8-get(ref)+ADs
+AEAAQA -368,4 +-388,4 +AEAAQA void percpu+AF8-ref+AF8-reinit(struct percpu+AF8-ref +ACo-ref)
spin+AF8-unlock+AF8-irqrestore(+ACY-percpu+AF8-ref+AF8-switch+AF8-lock, flags)+ADs
+AH0
-EXPORT+AF8-SYMBOL+AF8-GPL(percpu+AF8-ref+AF8-reinit)+ADs
+-EXPORT+AF8-SYMBOL+AF8-GPL(percpu+AF8-ref+AF8-resurrect)+ADs-