> Il giorno 05 mar 2018, alle ore 04:48, Konstantin Khlebnikov > <[email protected]> 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 <[email protected]> > --- > 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: >
