linux-kernel - Re: [PATCH] net: tcp: Fix a PTO timing granularity issue

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Message-ID: <1432743816.4060.404.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 27 May 2015 09:23:36 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ido Yariv <ido@...ery.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Nandita Dukkipati <nanditad@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ido Yariv <idox.yariv@...el.com>
Subject: Re: [PATCH] net: tcp: Fix a PTO timing granularity issue

On Wed, 2015-05-27 at 11:23 -0400, Ido Yariv wrote:

> Signed-off-by: Ido Yariv <idox.yariv@...el.com>
> ---
>  include/net/tcp.h     | 9 +++++++++
>  net/ipv4/tcp_output.c | 2 +-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 2bb2bad..86090b6 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1751,4 +1751,13 @@ static inline void skb_set_tcp_pure_ack(struct sk_buff *skb)
>  	skb->truesize = 2;
>  }
>  
> +/* Convert msecs to jiffies, ensuring that the return value is always at least
> + * 2. This can be used when setting tick-based timers to guarantee that they
> + * won't expire right away.
> + */
> +static inline unsigned long tcp_safe_msecs_to_jiffies(const unsigned int m)
> +{
> +	return max_t(u32, 2, msecs_to_jiffies(m));
> +}

Note that you can do slightly better if m is a constant at compile
time ;)

if (__builtin_constant_p(m) && m * 1000 >= 2 * HZ)
	return msecs_to_jiffies(m);
return max_t(u32, 2, msecs_to_jiffies(m));

Or something like that ?


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