linux-kernel - Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator

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Date:   Tue, 4 Apr 2017 12:42:24 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Bart Van Assche <bart.vanassche@...disk.com>
Cc:     "tj@...nel.org" <tj@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "ulf.hansson@...aro.org" <ulf.hansson@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "fchecconi@...il.com" <fchecconi@...il.com>,
        Arianna Avanzini <avanzini.arianna@...il.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "broonie@...nel.org" <broonie@...nel.org>
Subject: Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator


> Il giorno 31 mar 2017, alle ore 17:31, Bart Van Assche <bart.vanassche@...disk.com> ha scritto:
> 
> On Fri, 2017-03-31 at 14:47 +0200, Paolo Valente wrote:
>> -static bool bfq_update_peak_rate(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>> -                                bool compensate)
>> +static bool bfq_bfqq_is_slow(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>> +                                bool compensate, enum bfqq_expiration reason,
>> +                                unsigned long *delta_ms)
>>  {
>> -       u64 bw, usecs, expected, timeout;
>> -       ktime_t delta;
>> -       int update = 0;
>> +       ktime_t delta_ktime;
>> +       u32 delta_usecs;
>> +       bool slow = BFQQ_SEEKY(bfqq); /* if delta too short, use seekyness */
>>  
>> -       if (!bfq_bfqq_sync(bfqq) || bfq_bfqq_budget_new(bfqq))
>> +       if (!bfq_bfqq_sync(bfqq))
>>                 return false;
>>  
>>         if (compensate)
>> -               delta = bfqd->last_idling_start;
>> +               delta_ktime = bfqd->last_idling_start;
>>         else
>> -               delta = ktime_get();
>> -       delta = ktime_sub(delta, bfqd->last_budget_start);
>> -       usecs = ktime_to_us(delta);
>> -
>> -       /* Don't trust short/unrealistic values. */
>> -       if (usecs < 100 || usecs >= LONG_MAX)
>> -               return false;
>> -
>> -       /*
>> -        * Calculate the bandwidth for the last slice.  We use a 64 bit
>> -        * value to store the peak rate, in sectors per usec in fixed
>> -        * point math.  We do so to have enough precision in the estimate
>> -        * and to avoid overflows.
>> -        */
>> -       bw = (u64)bfqq->entity.service << BFQ_RATE_SHIFT;
>> -       do_div(bw, (unsigned long)usecs);
>> +               delta_ktime = ktime_get();
>> +       delta_ktime = ktime_sub(delta_ktime, bfqd->last_budget_start);
>> +       delta_usecs = ktime_to_us(delta_ktime);
>> +
> 
> This patch changes the type of the variable in which the result of ktime_to_us()
> is stored from u64 into u32 and next compares that result with LONG_MAX. Since
> ktime_to_us() returns a signed 64-bit number, are you sure you want to store that
> result in a 32-bit variable? If ktime_to_us() would e.g. return 0xffffffff00000100
> or 0x100000100 then the assignment will truncate these numbers to 0x100.
> 

The instruction above the assignment you highlight stores in
delta_ktime the difference between 'now' and the last budget start.
The latter may have happened at most about 100 ms before 'now'.  So
there should be no overflow issue.

Thanks,
Paolo

> Bart.