Messages in this thread | | | Subject | Re: [linux-pm] [PATCH 0/8] Suspend block api (version 6) | From | Kevin Hilman <> | Date | Mon, 17 May 2010 08:40:59 -0700 |
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Alan Stern <stern@rowland.harvard.edu> writes:
> On Fri, 14 May 2010, Brian Swetland wrote: > >> In tickless mode, the time until next timer is a signed int, so the >> longest the kernel will ever sleep is ~2 seconds at a go. In >> practice, userspace entities often have polling behavior that can >> trigger more often than that, and I've observed some kernel periodic >> timers (haven't cataloged them recently) that happen more often than >> once a second. > > Paul and Kevin, how does the OMAP implementation handle these > difficulties?
just a minor clarification... these aren't OMAP-specific issues, but generic issues to all power-sensitive kernel users.
The ~2 second limit was fixed by Jon Hunter (TI) and is in mainline since 2.6.32[1].
For other timers, there has been active work (mostly by Intel folks) on deferrable timers, coalescing timers, timer slack etc. that has greatly reduced the kernel timer impact on wakeups.
> Also, how does it handle the issue of ill-behaved apps?
For userspace, apps that have polling behavior or are ill-behaved must be found and fixed. Thanks to tools like powertop, this is a farily easy task.
But really, I don't consider the "ill-behaved app" problem to be a real-world problem. Both in maemo/meego and Android, if someone writes an app that kills battery life, it will get reported as a bug, or get bad ratings etc. On these kinds of devices, there is a *stong* developer incentive to not write battery sucking apps.
Kevin
[1] commit 97813f2fe77804a4464564c75ba8d8826377feea Author: Jon Hunter <jon-hunter@ti.com> Date: Tue Aug 18 12:45:11 2009 -0500
nohz: Allow 32-bit machines to sleep for more than 2.15 seconds
In the dynamic tick code, "max_delta_ns" (member of the "clock_event_device" structure) represents the maximum sleep time that can occur between timer events in nanoseconds.
The variable, "max_delta_ns", is defined as an unsigned long which is a 32-bit integer for 32-bit machines and a 64-bit integer for 64-bit machines (if -m64 option is used for gcc). The value of max_delta_ns is set by calling the function "clockevent_delta2ns()" which returns a maximum value of LONG_MAX. For a 32-bit machine LONG_MAX is equal to 0x7fffffff and in nanoseconds this equates to ~2.15 seconds. Hence, the maximum sleep time for a 32-bit machine is ~2.15 seconds, where as for a 64-bit machine it will be many years. This patch changes the type of max_delta_ns to be "u64" instead of "unsigned long" so that this variable is a 64-bit type for both 32-bit and 64-bit machines. It also changes the maximum value returned by clockevent_delta2ns() to KTIME_MAX. Hence this allows a 32-bit machine to sleep for longer than ~2.15 seconds. Please note that this patch also changes "min_delta_ns" to be "u64" too and although this is unnecessary, it makes the patch simpler as it avoids to fixup all callers of clockevent_delta2ns(). [ tglx: changed "unsigned long long" to u64 as we use this data type through out the time code ] Signed-off-by: Jon Hunter <jon-hunter@ti.com> Cc: John Stultz <johnstul@us.ibm.com> LKML-Reference: <1250617512-23567-3-git-send-email-jon-hunter@ti.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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