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

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 19 mar 2018, alle ore 14:28, Konstantin Khlebnikov <khlebnikov@yandex-team.ru> ha scritto:
> 
> 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..
> 

Yep.  Actually Murphy has been even clement this time, by making the
failure occur to a kernel expert :)

> 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. =)
> 

I see your point.  Still, if the mistake is not in sizing, then you
bumped into some odd bug.  In this respect, I don't like much the idea
of sweeping the dust under the carpet.  So, let me ask you for a
little bit more help.  With your patch applied, and thus with no risk
of crashes, what about adding, right before your clamp_t, something
like:

if (!bfqd->peak_rate)
	pr_crit(<dump of all the variables involved in updating bfqd->peak_rate>);

Once the failure shows up (Murphy permitting), we might have hints to
the bug causing it.

Apart from that, I have no problem with patches that make bfq more
robust, even in a sort of black-box way.

Thanks a lot,
Paolo

> 
>>> 	update_thr_responsiveness_params(bfqd);
>>> 
>>> reset_computation: