Messages in this thread | | | Subject | Re: [PATCH] block, bfq: keep peak_rate estimation within range 1..2^32-1 | From | Konstantin Khlebnikov <> | Date | Mon, 19 Mar 2018 09:28:25 +0300 |
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On 19.03.2018 09:03, Paolo Valente wrote: > > >> Il giorno 05 mar 2018, alle ore 04:48, Konstantin Khlebnikov <khlebnikov@yandex-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@yandex-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?
According to Murphy's law this is inevitable..
I've seen couple division by zero crashes in bfq_wr_duration. Unfortunately logs weren't recorded.
> > 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? >>> + bfqd->peak_rate = clamp_t(u64, rate + bfqd->peak_rate, 1, U32_MAX);
32-bit should be enough and better for division. My patch makes sure it never overflows/underflows. That's cheaper than full 64-bit/64-bit division. Anyway 64-bit speed could overflow too. =)
>> update_thr_responsiveness_params(bfqd); >> >> reset_computation: >> >
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