Re: [PATCH 4/7] blk-iocost: Add a flag to indicate if need update hwi

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Hello,

On Tue, Nov 24, 2020 at 11:33:33AM +0800, Baolin Wang wrote:
@@ -1445,7 +1447,8 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt,
  	 * after the above debt payment.
  	 */
  	ctx.vbudget = vbudget;
-	current_hweight(iocg, NULL, &ctx.hw_inuse);
+	if (need_update_hwi)
+		current_hweight(iocg, NULL, &ctx.hw_inuse);

So, if you look at the implementation of current_hweight(), it's

1. If nothing has changed, read out the cached values.
2. If something has changed, recalculate.

Yes, correct.


and the "something changed" test is single memory read (most likely L1 hot
at this point) and testing for equality. IOW, the change you're suggesting
isn't much of an optimization. Maybe the compiler can do a somewhat better
job of arranging the code and it's a register load than memory load but
given that it's already a relatively cold wait path, this is unlikely to
make any actual difference. And that's how current_hweight() is meant to be
used.

What I want to avoid is the 'atomic_read(&ioc->hweight_gen)' in current_hweight(), cause this is not a register load and is always a memory load. But introducing a flag can be cached and more light than a memory load.

But after thinking more, I think we can just move the "current_hweight(iocg, NULL, &ctx.hw_inuse);" to the correct place without introducing new flag to optimize the code. How do you think the below code?

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index bbe86d1..db29200 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -1413,7 +1413,7 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt,

        lockdep_assert_held(&iocg->waitq.lock);

-       current_hweight(iocg, &hwa, NULL);
+       current_hweight(iocg, &hwa, &ctx.hw_inuse);
        vbudget = now->vnow - atomic64_read(&iocg->vtime);

        /* pay off debt */
@@ -1428,6 +1428,11 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt,
                atomic64_add(vpay, &iocg->done_vtime);
                iocg_pay_debt(iocg, abs_vpay, now);
                vbudget -= vpay;
+               /*
+ * As paying off debt restores hw_inuse, it must be read after
+                * the above debt payment.
+                */
+               current_hweight(iocg, NULL, &ctx.hw_inuse);
        }

        if (iocg->abs_vdebt || iocg->delay)
@@ -1446,11 +1451,9 @@ static void iocg_kick_waitq(struct ioc_gq *iocg, bool pay_debt,

        /*
* Wake up the ones which are due and see how much vtime we'll need for - * the next one. As paying off debt restores hw_inuse, it must be read
-        * after the above debt payment.
+        * the next one.
         */
        ctx.vbudget = vbudget;
-       current_hweight(iocg, NULL, &ctx.hw_inuse);

        __wake_up_locked_key(&iocg->waitq, TASK_NORMAL, &ctx);


So, I'm not sure this is an improvement. It increases complication without
actually gaining anything.

Thanks.




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