Messages in this thread Patch in this message | | | Subject | Re: [PATCH 4/7] blk-iocost: Add a flag to indicate if need update hwi | From | Baolin Wang <> | Date | Wed, 25 Nov 2020 22:15:38 +0800 |
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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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