linux-kernel - Re: [PATCH] block, bfq: keep peak_rate estimation within range 1..2^32-1

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Date:   Sun, 18 Mar 2018 23:03:43 -0700
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-block <linux-block@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        'Paolo Valente' via bfq-iosched 
        <bfq-iosched@...glegroups.com>
Subject: Re: [PATCH] block, bfq: keep peak_rate estimation within range
 1..2^32-1

> Il giorno 05 mar 2018, alle ore 04:48, Konstantin Khlebnikov <khlebnikov@...dex-team.ru> ha scritto:
> 
> Rate should never overflow or become zero because it is used as divider.
> This patch accumulates it with saturation.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> ---
> block/bfq-iosched.c |    8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index aeca22d91101..a236c8d541b5 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2546,7 +2546,8 @@ static void bfq_reset_rate_computation(struct bfq_data *bfqd,
> 
> static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq)
> {
> -	u32 rate, weight, divisor;
> +	u32 weight, divisor;
> +	u64 rate;
> 
> 	/*
> 	 * For the convergence property to hold (see comments on
> @@ -2634,9 +2635,10 @@ static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq)
> 	 */
> 	bfqd->peak_rate *= divisor-1;
> 	bfqd->peak_rate /= divisor;
> -	rate /= divisor; /* smoothing constant alpha = 1/divisor */
> +	do_div(rate, divisor);	/* smoothing constant alpha = 1/divisor */
> 
> -	bfqd->peak_rate += rate;
> +	/* rate should never overlow or become zero */

It is bfqd->peak_rate that is used as a divider, and bfqd->peak_rate doesn't risk to be zero even if the variable 'rate' is zero here.

So I guess the reason why you consider the possibility that bfqd->peak_rate becomes zero is because of an overflow when summing 'rate'. But, according to my calculations, this should be impossible with devices with sensible speeds.

These are the reasons why I decided I could make it with a 32-bit variable, without any additional clamping. Did I make any mistake in my evaluation?

Anyway, even if I made some mistake about the maximum possible value of the device rate, and the latter may be too high for bfqd->peak_rate to contain it, then I guess the right solution would not be to clamp the actual rate to U32_MAX, but to move bfqd->peak_rate to 64 bits. Or am I missing something else?

Thanks,
Paolo

> +	bfqd->peak_rate = clamp_t(u64, rate + bfqd->peak_rate, 1, U32_MAX);
> 	update_thr_responsiveness_params(bfqd);
> 
> reset_computation:
>