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

[Baolin Wang]

> 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.