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

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Date:   Mon, 05 Mar 2018 15:48:53 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Paolo Valente <paolo.valente@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] block, bfq: keep peak_rate estimation within range 1..2^32-1

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@...dex-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 */
+	bfqd->peak_rate = clamp_t(u64, rate + bfqd->peak_rate, 1, U32_MAX);
 	update_thr_responsiveness_params(bfqd);
 
 reset_computation: