netdev - Re: [PATCH AUTOSEL 5.4 07/26] net sched: fix reporting the first-time use timestamp

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Date:   Sun, 31 May 2020 23:19:25 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Roman Mashak <mrv@...atatu.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.4 07/26] net sched: fix reporting the
 first-time use timestamp

Hi!

> From: Roman Mashak <mrv@...atatu.com>
> 
> [ Upstream commit b15e62631c5f19fea9895f7632dae9c1b27fe0cd ]
> 
> When a new action is installed, firstuse field of 'tcf_t' is explicitly set
> to 0. Value of zero means "new action, not yet used"; as a packet hits the
> action, 'firstuse' is stamped with the current jiffies value.
> 
> tcf_tm_dump() should return 0 for firstuse if action has not yet been hit.

> @@ -69,7 +69,8 @@ static inline void tcf_tm_dump(struct tcf_t *dtm, const struct tcf_t *stm)
>  {
>  	dtm->install = jiffies_to_clock_t(jiffies - stm->install);
>  	dtm->lastuse = jiffies_to_clock_t(jiffies - stm->lastuse);
> -	dtm->firstuse = jiffies_to_clock_t(jiffies - stm->firstuse);
> +	dtm->firstuse = stm->firstuse ?
> +		jiffies_to_clock_t(jiffies - stm->firstuse) : 0;
>  	dtm->expires = jiffies_to_clock_t(stm->expires);
>  }

Jiffies start at small negative value (and are expected to overflow
periodically). This code will not work correctly in that case.

Best regards,
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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