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From: Paolo Valente <email@example.com>
To: Konstantin Khlebnikov <firstname.lastname@example.org>
Cc: linux-block <email@example.com>,
Jens Axboe <firstname.lastname@example.org>,
Linux Kernel Mailing List <email@example.com>,
"'Paolo Valente' via bfq-iosched" <firstname.lastname@example.org>,
Mark Brown <email@example.com>,
Ulf Hansson <firstname.lastname@example.org>,
Linus Walleij <email@example.com>
Subject: Re: [PATCH] block, bfq: keep peak_rate estimation within range 1..2^32-1
Date: Tue, 20 Mar 2018 11:00:11 +0800 [thread overview]
Message-ID: <E9AC28A6-848D-420F-8DAB-8B1FC709BA38@linaro.org> (raw)
> Il giorno 19 mar 2018, alle ore 14:28, Konstantin Khlebnikov <firstname.lastname@example.org> ha scritto:
> On 19.03.2018 09:03, Paolo Valente wrote:
>>> Il giorno 05 mar 2018, alle ore 04:48, Konstantin Khlebnikov <email@example.com> 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 <firstname.lastname@example.org>
>>> 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
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,
next prev parent reply other threads:[~2018-03-20 3:00 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-05 12:48 Konstantin Khlebnikov
2018-03-19 6:03 ` Paolo Valente
2018-03-19 6:28 ` Konstantin Khlebnikov
2018-03-20 3:00 ` Paolo Valente [this message]
2018-03-20 7:41 ` Konstantin Khlebnikov
2018-03-20 23:49 ` Paolo Valente
2018-03-26 8:01 ` Paolo Valente
2018-03-26 10:28 ` Konstantin Khlebnikov
2018-03-26 14:06 ` Paolo Valente
2018-03-26 14:18 ` Konstantin Khlebnikov
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