linux-kernel - Re: [PATCH v2 04/10] blk-throttle: correct calculation of wait time in tg_may

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Date:   Wed, 30 Nov 2022 11:27:21 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Kemeng Shi <shikemeng@...wei.com>
Cc:     josef@...icpanda.com, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/10] blk-throttle: correct calculation of wait time
 in tg_may_dispatch

On Tue, Nov 29, 2022 at 11:01:41AM +0800, Kemeng Shi wrote:
> In C language, When executing "if (expression1 && expression2)" and
> expression1 return false, the expression2 may not be executed.
> For "tg_within_bps_limit(tg, bio, bps_limit, &bps_wait) &&
> tg_within_iops_limit(tg, bio, iops_limit, &iops_wait))", if bps is
> limited, tg_within_bps_limit will return false and
> tg_within_iops_limit will not be called. So even bps and iops are
> both limited, iops_wait will not be calculated and is always zero.
> So wait time of iops is always ignored.
> 
> Fix this by always calling tg_within_bps_limit and tg_within_iops_limit
> to get wait time for both bps and iops.
> 
> Observed that:
> 1. Wait time in tg_within_iops_limit/tg_within_bps_limit need always
> be stored as wait argument is always passed.
> 2. wait time is stored to zero if iops/bps is limited otherwise non-zero
> is stored.
> Simpfy tg_within_iops_limit/tg_within_bps_limit by removing wait argument
> and return wait time directly. Caller tg_may_dispatch checks if wait time
> is zero to find if iops/bps is limited.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...wei.com>

Acked-by: Tejun Heo <tj@...nel.org>

-- 
tejun