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Date: Wed, 12 Oct 2011 00:02:49 +0200 From: "Rafael J. Wysocki" <rjw@...k.pl> To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> Cc: rdunlap@...otime.net, len.brown@...el.com, pavel@....cz, a.p.zijlstra@...llo.nl, linux-pm@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] Documentation/power: Update docs about suspend and CPU hotplug On Tuesday, October 11, 2011, Srivatsa S. Bhat wrote: > Update the documentation about the interaction between the suspend (S3) call > path and the CPU hotplug infrastructure. > This patch focusses only on the activities of the freezer, cpu hotplug and > the notifications involved. It outlines how regular CPU hotplug differs from > the way it is invoked during suspend and also tries to explain the locking > involved. > > Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com> > --- > > Documentation/power/00-INDEX | 2 > Documentation/power/suspend-and-cpuhotplug.txt | 113 ++++++++++++++++++++++++ > 2 files changed, 115 insertions(+), 0 deletions(-) > create mode 100644 Documentation/power/suspend-and-cpuhotplug.txt > > diff --git a/Documentation/power/00-INDEX b/Documentation/power/00-INDEX > index 45e9d4a..a4d682f 100644 > --- a/Documentation/power/00-INDEX > +++ b/Documentation/power/00-INDEX > @@ -26,6 +26,8 @@ s2ram.txt > - How to get suspend to ram working (and debug it when it isn't) > states.txt > - System power management states > +suspend-and-cpuhotplug.txt > + - Explains the interaction between Suspend-to-RAM (S3) and CPU hotplug > swsusp-and-swap-files.txt > - Using swap files with software suspend (to disk) > swsusp-dmcrypt.txt > diff --git a/Documentation/power/suspend-and-cpuhotplug.txt b/Documentation/power/suspend-and-cpuhotplug.txt > new file mode 100644 > index 0000000..d0ba411 > --- /dev/null > +++ b/Documentation/power/suspend-and-cpuhotplug.txt > @@ -0,0 +1,113 @@ > +Interaction of Suspend code (S3) with the CPU hotplug infrastructure > + (C) 2011 Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>, GPL > + > + > +I. How does the Suspend-to-RAM code interact with CPU hotplug infrastructure? > + > +Well, a picture speaks more than a thousand words... So ASCII art follows :-) > + > +[This depicts the current design in the kernel, and focusses only on the > +interactions between suspend call paths involving the freezer and cpu hotplug > +and also tries to explain the locking involved. It also outlines the > +notifications involved.] > + > +On a high level, the suspend-resume cycle goes like this: > + > +|Freeze| -> |Disable nonboot| -> |Do suspend| -> |Enable nonboot| -> |Thaw | > +|tasks | | cpus | | | | cpus | |tasks| > + > + > +More details follow: > + > +Regular CPU hotplug Suspend call path > +------------------- --------------------------- > + > +Write 0 (or 1) to Write 'mem' to > +/sys/devices/system/cpu/cpu*/online /sys/power/state > + sysfs file syfs file > + | | > + | v > + | Acquire pm_mutex lock > + | | > + | v > + | Send PM_SUSPEND_PREPARE notifications > + | | > + | v > + | Freeze tasks OK, so something appears to be missing here. Namely, the task writing to /sys/devices/system/cpu/cpu*/online should be frozen at this point or suspend should be aborted. I suppose neither of these happens and I wonder why exactly. Thanks, Rafael > + | | > + | | > + v v > + cpu_down() disable_nonboot_cpus() /*start*/ > + | | > + v v > +Acquire cpu_add_remove_lock Acquire cpu_add_remove_lock > + | | > + v v > +If cpu_hotplug_disabled is 1 Iterate over CURRENTLY online CPUs > + return gracefully | > + | | > + | | ---- > + v v | > + \ / | > + -------- -------- | > + \ / | > + -------- -------- |L > + \____/ | > + | | > + v |O > + _cpu_down() | > + [This takes cpuhotplug.lock | > + before taking down the CPU | > + and releases it when done] |O > + While it is at it, notifications | > + are sent when notable events occur, | > + by running all registered callbacks. | > + | |O > + / \ | > + / \ | > + < > | > + _______________________/ \_____________________ |P > + | | | > + v v | > +Release cpu_add_remove_lock Note down these cpus in | > +[That's it!, for frozen_cpus mask ---- > + regular CPU hotplug] | > + v > + Disable regular cpu hotplug > + by setting cpu_hotplug_disabled=1 > + | > + v > + Release cpu_add_remove_lock > + | > + v > + /* disable_nonboot_cpus() complete */ > + | > + v > + Do suspend > + > + > +Resuming back is likewise, with the counterparts being (in the order of > +execution during resume): > +* enable_nonboot_cpus() which involves: > + | Acquire cpu_add_remove_lock > + | Reset cpu_hotplug_disabled to 0, thereby enabling regular cpu hotplug > + | Call _cpu_up() [for all those cpus in the frozen_cpus mask, in a loop] > + | Release cpu_add_remove_lock > + v > + > +* thaw tasks > +* send PM_POST_SUSPEND notifications > +* Release pm_mutex lock. > + > +It is to be noted here that the pm_mutex lock is acquired at the very > +beginning, when we are just starting out to suspend, and then released only > +after the entire cycle is complete (i.e., suspend + resume). > + > + > +Important files and functions/entry points: > +------------------------------------------ > + > +kernel/power/process.c : freeze_processes(), thaw_processes() > +kernel/power/suspend.c : suspend_prepare(), suspend_enter(), suspend_finish() > +kernel/cpu.c: cpu_[up|down](), _cpu_[up|down](), [disable|enable]_nonboot_cpus() > + > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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