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

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Date:   Wed, 25 Nov 2020 07:14:48 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     axboe@...nel.dk, baolin.wang7@...il.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] blk-iocost: Add a flag to indicate if need update hwi

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.

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.

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

Thanks.

-- 
tejun