> Il giorno 05 mar 2018, alle ore 04:48, Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> ---
> 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:
>
