linux-kernel - Re: [PATCH] blk-throttle: Calculate allowed value only when the throttle is enabled

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Message-ID: <20231008113602.GB24726@redhat.com>
Date:   Sun, 8 Oct 2023 13:36:03 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     Li Nan <linan666@...weicloud.com>,
        Khazhy Kumykov <khazhy@...omium.org>, tj@...nel.org,
        josef@...icpanda.com, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, houtao1@...wei.com, yangerkun@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled

On 10/08, Yu Kuai wrote:
>
> >>  static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long
> >>jiffy_elapsed)
> >>  {
> >>+       if (jiffy_elapsed > HZ &&
> >>+           bps_limit > mul_u64_u64_div_u64(U64_MAX, (u64)HZ,
> >>(u64)jiffy_elapsed);
> >>+               return U64_MAX;
> >>+
> >
> >I can't suggest anything better...
> >
> >but I do not know if it is possible that HZ > jiffy_elapsed. If yes, then
> >mul_u64_u64_div_u64() above is not safe too.
>
> Well, 'jiffy_elapsed > HZ' is judged before mul_u64_u64_div_u64().

Yes, sorry, somehow I didn't notice this check.

Oleg.