From: Thomas Gleixner <tglx@linutronix.de>
To: Artem Savkov <asavkov@redhat.com>,
jpoimboe@redhat.com, netdev@vger.kernel.org
Cc: davem@davemloft.net, yoshfuji@linux-ipv6.org, dsahern@kernel.org,
linux-kernel@vger.kernel.org, Artem Savkov <asavkov@redhat.com>,
Anna-Maria Gleixner <anna-maria@linutronix.de>
Subject: Re: [PATCH 1/2] timer: introduce upper bound timers
Date: Thu, 24 Mar 2022 14:54:21 +0100 [thread overview]
Message-ID: <87h77ncv76.ffs@tglx> (raw)
In-Reply-To: <87tubn8rgk.ffs@tglx>
On Thu, Mar 24 2022 at 13:28, Thomas Gleixner wrote:
> On Wed, Mar 23 2022 at 12:16, Artem Savkov wrote:
>> Add TIMER_UPPER_BOUND flag which allows creation of timers that would
>> expire at most at specified time or earlier.
>>
>> This was previously discussed here:
>> https://lore.kernel.org/all/20210302001054.4qgrvnkltvkgikzr@treble/T/#u
>
> please add the context to the changelog. A link is only supplemental
> information and does not replace content.
>
>> static inline unsigned calc_index(unsigned long expires, unsigned lvl,
>> - unsigned long *bucket_expiry)
>> + unsigned long *bucket_expiry, bool upper_bound)
>> {
>>
>> /*
>> @@ -501,34 +501,39 @@ static inline unsigned calc_index(unsigned long expires, unsigned lvl,
>> * - Truncation of the expiry time in the outer wheel levels
>> *
>> * Round up with level granularity to prevent this.
>> + * Do not perform round up in case of upper bound timer.
>> */
>> - expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
>> + if (upper_bound)
>> + expires = expires >> LVL_SHIFT(lvl);
>> + else
>> + expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
>
> While this "works", I fundamentally hate this because it adds an extra
> conditional into the common case. That affects every user of the timer
> wheel. We went great length to optimize that code and I'm not really enthused
> to sacrifice that just because of _one_ use case.
Aside of that this is not mathematically correct. Why?
The level selection makes the cutoff at: LEVEL_MAX(lvl) - 1. E.g. 62
instead of 63 for the first level.
The reason is that this accomodates for the + LVL_GRAN(lvl). Now with
surpressing the roundup this creates a gap. Not a horrible problem, but
not correct either.
Thanks,
tglx
next prev parent reply other threads:[~2022-03-24 13:54 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-23 11:16 [PATCH 0/2] Upper bound mode for kernel timers Artem Savkov
2022-03-23 11:16 ` [PATCH 1/2] timer: introduce upper bound timers Artem Savkov
2022-03-23 18:40 ` Josh Poimboeuf
2022-03-24 9:14 ` [PATCH v2 0/2] Upper bound mode for kernel timers Artem Savkov
2022-03-24 9:14 ` [PATCH v2 1/2] timer: introduce upper bound timers Artem Savkov
2022-03-24 9:15 ` [PATCH v2 2/2] net: make tcp keepalive timer upper bound Artem Savkov
2022-03-24 12:28 ` [PATCH 1/2] timer: introduce upper bound timers Thomas Gleixner
2022-03-24 13:54 ` Thomas Gleixner [this message]
2022-03-26 21:13 ` Thomas Gleixner
2022-03-30 8:20 ` [PATCH v3 0/2] Upper bound kernel timers Artem Savkov
2022-03-30 8:20 ` [PATCH v3 1/2] timer: add a function to adjust timeouts to be upper bound Artem Savkov
2022-03-30 13:40 ` Anna-Maria Behnsen
2022-04-02 6:55 ` Artem Savkov
2022-04-05 15:33 ` Thomas Gleixner
2022-04-07 7:52 ` [PATCH v4 0/2] Upper bound kernel timers Artem Savkov
2022-04-07 7:52 ` [PATCH v4 1/2] timer: add a function to adjust timeouts to be upper bound Artem Savkov
2022-04-08 0:37 ` Thomas Gleixner
2022-04-08 5:39 ` Josh Poimboeuf
2022-04-12 13:42 ` Artem Savkov
2022-05-05 13:18 ` [PATCH v5 0/2] Upper bound kernel timers Artem Savkov
2022-05-05 13:18 ` [PATCH v5 1/2] timer: add a function to adjust timeouts to be upper bound Artem Savkov
2022-05-05 13:18 ` [PATCH v5 2/2] net: make tcp keepalive timer " Artem Savkov
2022-05-05 17:56 ` Josh Poimboeuf
2022-05-06 6:39 ` Artem Savkov
2022-05-06 16:24 ` Josh Poimboeuf
2022-07-26 22:42 ` [PATCH v5 0/2] Upper bound kernel timers Josh Poimboeuf
2022-04-07 7:52 ` [PATCH v4 2/2] net: make tcp keepalive timer upper bound Artem Savkov
[not found] ` <Yk1i3WrcVIICAiF0@samus.usersys.redhat.com>
2022-04-07 23:26 ` [PATCH v3 1/2] timer: add a function to adjust timeouts to be " Thomas Gleixner
2022-03-30 8:20 ` [PATCH v3 2/2] net: make tcp keepalive timer " Artem Savkov
2022-04-02 3:09 ` [net] 6ef3f95797: UBSAN:shift-out-of-bounds_in_kernel/time/timer.c kernel test robot
2022-04-02 7:11 ` Artem Savkov
2022-03-30 10:28 ` [PATCH v3 0/2] Upper bound kernel timers David Laight
2022-03-25 7:38 ` [timer] d41e0719d5: UBSAN:shift-out-of-bounds_in_lib/flex_proportions.c kernel test robot
2022-03-25 19:14 ` Thomas Gleixner
2022-03-23 11:16 ` [PATCH 2/2] net: make tcp keepalive timer upper bound Artem Savkov
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