Hi Eric, On Wed, May 27, 2015 at 03:15:26PM -0400, Ido Yariv wrote: > Hi Eric, > > On Wed, May 27, 2015 at 10:24:16AM -0700, Eric Dumazet wrote: > > On Wed, 2015-05-27 at 12:54 -0400, Ido Yariv wrote: > > > Hi Eric, > > > > > That's a nice optimization ;) > > > > > > However, I think that with Nicholas Mc Guire's recent changes to > > > msecs_to_jiffies (http://marc.info/?l=linux-kernel&m=143195210010666), > > > we should get this for free, no? > > > > Well, on net and net-next tree we currently have : > > > > $ grep msecs_to_jiffies include/linux/jiffies.h > > extern unsigned long msecs_to_jiffies(const unsigned int m); > > > > Given your patch is for stable, I would not mind having this done > > anyway. > > I believe these changes are in tip, but not in net/net-next just yet. > > I actually didn't think this patch is for stable, but we can certainly > do that. > > Would you be fine with the patch below? Please note that I modified your > optimization a bit.
We'd probably like to avoid any potential integer overflows as well, so I modified the if statement accordingly. Not sure this optimization is really beneficial (and only on stable kernels), but here's the updated patch. Cheers, Ido. >From 8d78f75cde8ef523a312e6cfab8e1b7a89f97c9f Mon Sep 17 00:00:00 2001 From: Ido Yariv <i...@wizery.com> Date: Thu, 21 May 2015 08:23:13 +0200 Subject: [PATCH v5] net: tcp: Fix a PTO timing granularity issue The Tail Loss Probe RFC specifies that the PTO value should be set to max(2 * SRTT, 10ms), where SRTT is the smoothed round-trip time. The PTO value is converted to jiffies, so the timer may expire prematurely. This is especially problematic on systems in which HZ <= 100, so work around this by setting the timeout to at least 2 jiffies on such systems. The 10ms figure was originally selected based on tests performed with the current implementation and HZ = 1000. Thus, leave the behavior on systems with HZ > 100 unchanged. Signed-off-by: Ido Yariv <idox.ya...@intel.com> --- include/net/tcp.h | 20 ++++++++++++++++++++ net/ipv4/tcp_output.c | 2 +- 2 files changed, 21 insertions(+), 1 deletion(-) diff --git a/include/net/tcp.h b/include/net/tcp.h index 2bb2bad..19ed4c0 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1751,4 +1751,24 @@ 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 at least 2 + * jiffies. + * 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) +{ + if (__builtin_constant_p(m)) { + /* The theoretical upper bound of m for 2 jiffies is 2 seconds, + * so compare m with that to avoid potential integer overflows. + */ + if ((m > 2 * MSEC_PER_SEC) || (m * HZ > 2 * MSEC_PER_SEC)) + return msecs_to_jiffies(m); + + return 2; + } + + return max_t(u32, 2, msecs_to_jiffies(m)); +} + #endif /* _TCP_H */ diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 190538a..26cc5a6 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2207,7 +2207,7 @@ bool tcp_schedule_loss_probe(struct sock *sk) if (tp->packets_out == 1) timeout = max_t(u32, timeout, (rtt + (rtt >> 1) + TCP_DELACK_MAX)); - timeout = max_t(u32, timeout, msecs_to_jiffies(10)); + timeout = max_t(u32, timeout, tcp_safe_msecs_to_jiffies(10)); /* If RTO is shorter, just schedule TLP in its place. */ tlp_time_stamp = tcp_time_stamp + timeout; -- 2.1.0 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html