On 19.03.2018 09:03, Paolo Valente wrote:
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?
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:
