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author | James Bottomley <JBottomley@Parallels.com> | 2012-05-21 12:17:30 +0100 |
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committer | James Bottomley <JBottomley@Parallels.com> | 2012-05-21 12:17:30 +0100 |
commit | e34693336564f02b3e2cc09d8b872aef22a154e9 (patch) | |
tree | 09f51f10f9406042f9176e39b4dc8de850ba712e /Documentation/power/freezing-of-tasks.txt | |
parent | 76b311fdbdd2e16e5d39cd496a67aa1a1b948914 (diff) | |
parent | de2eb4d5c5c25e8fb75d1e19092f24b83cb7d8d5 (diff) | |
download | linux-e34693336564f02b3e2cc09d8b872aef22a154e9.tar.bz2 |
Merge tag 'isci-for-3.5' into misc
isci update for 3.5
1/ Rework remote-node-context (RNC) handling for proper management of
the silicon state machine in error handling and hot-plug conditions.
Further details below, suffice to say if the RNC is mismanaged the
silicon state machines may lock up.
2/ Refactor the initialization code to be reused for suspend/resume support
3/ Miscellaneous bug fixes to address discovery issues and hardware
compatibility.
RNC rework details from Jeff Skirvin:
In the controller, devices as they appear on a SAS domain (or
direct-attached SATA devices) are represented by memory structures known
as "Remote Node Contexts" (RNCs). These structures are transferred from
main memory to the controller using a set of register commands; these
commands include setting up the context ("posting"), removing the
context ("invalidating"), and commands to control the scheduling of
commands and connections to that remote device ("suspensions" and
"resumptions"). There is a similar path to control RNC scheduling from
the protocol engine, which interprets the results of command and data
transmission and reception.
In general, the controller chooses among non-suspended RNCs to find one
that has work requiring scheduling the transmission of command and data
frames to a target. Likewise, when a target tries to return data back
to the initiator, the state of the RNC is used by the controller to
determine how to treat the incoming request. As an example, if the RNC
is in the state "TX/RX Suspended", incoming SSP connection requests from
the target will be rejected by the controller hardware. When an RNC is
"TX Suspended", it will not be selected by the controller hardware to
start outgoing command or data operations (with certain priority-based
exceptions).
As mentioned above, there are two sources for management of the RNC
states: commands from driver software, and the result of transmission
and reception conditions of commands and data signaled by the controller
hardware. As an example of the latter, if an outgoing SSP command ends
with a OPEN_REJECT(BAD_DESTINATION) status, the RNC state will
transition to the "TX Suspended" state, and this is signaled by the
controller hardware in the status to the completion of the pending
command as well as signaled in a controller hardware event. Examples of
the former are included in the patch changelogs.
Driver software is required to suspend the RNC in a "TX/RX Suspended"
condition before any outstanding commands can be terminated. Failure to
guarantee this can lead to a complete hardware hang condition. Earlier
versions of the driver software did not guarantee that an RNC was
correctly managed before I/O termination, and so operated in an unsafe
way.
Further, the driver performed unnecessary contortions to preserve the
remote device command state and so was more complicated than it needed
to be. A simplifying driver assumption is that once an I/O has entered
the error handler path without having completed in the target, the
requirement on the driver is that all use of the sas_task must end.
Beyond that, recovery of operation is dependent on libsas and other
components to reset, rediscover and reconfigure the device before normal
operation can restart. In the driver, this simplifying assumption meant
that the RNC management could be reduced to entry into the suspended
state, terminating the targeted I/O request, and resuming the RNC as
needed for device-specific management such as an SSP Abort Task or LUN
Reset Management request.
Diffstat (limited to 'Documentation/power/freezing-of-tasks.txt')
-rw-r--r-- | Documentation/power/freezing-of-tasks.txt | 37 |
1 files changed, 19 insertions, 18 deletions
diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt index ec715cd78fbb..6ec291ea1c78 100644 --- a/Documentation/power/freezing-of-tasks.txt +++ b/Documentation/power/freezing-of-tasks.txt @@ -9,7 +9,7 @@ architectures). II. How does it work? -There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE +There are three per-task flags used for that, PF_NOFREEZE, PF_FROZEN and PF_FREEZER_SKIP (the last one is auxiliary). The tasks that have PF_NOFREEZE unset (all user space processes and some kernel threads) are regarded as 'freezable' and treated in a special way before the system enters a @@ -17,30 +17,31 @@ suspend state as well as before a hibernation image is created (in what follows we only consider hibernation, but the description also applies to suspend). Namely, as the first step of the hibernation procedure the function -freeze_processes() (defined in kernel/power/process.c) is called. It executes -try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and -either wakes them up, if they are kernel threads, or sends fake signals to them, -if they are user space processes. A task that has TIF_FREEZE set, should react -to it by calling the function called __refrigerator() (defined in -kernel/freezer.c), which sets the task's PF_FROZEN flag, changes its state -to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it. -Then, we say that the task is 'frozen' and therefore the set of functions -handling this mechanism is referred to as 'the freezer' (these functions are -defined in kernel/power/process.c, kernel/freezer.c & include/linux/freezer.h). -User space processes are generally frozen before kernel threads. +freeze_processes() (defined in kernel/power/process.c) is called. A system-wide +variable system_freezing_cnt (as opposed to a per-task flag) is used to indicate +whether the system is to undergo a freezing operation. And freeze_processes() +sets this variable. After this, it executes try_to_freeze_tasks() that sends a +fake signal to all user space processes, and wakes up all the kernel threads. +All freezable tasks must react to that by calling try_to_freeze(), which +results in a call to __refrigerator() (defined in kernel/freezer.c), which sets +the task's PF_FROZEN flag, changes its state to TASK_UNINTERRUPTIBLE and makes +it loop until PF_FROZEN is cleared for it. Then, we say that the task is +'frozen' and therefore the set of functions handling this mechanism is referred +to as 'the freezer' (these functions are defined in kernel/power/process.c, +kernel/freezer.c & include/linux/freezer.h). User space processes are generally +frozen before kernel threads. __refrigerator() must not be called directly. Instead, use the try_to_freeze() function (defined in include/linux/freezer.h), that checks -the task's TIF_FREEZE flag and makes the task enter __refrigerator() if the -flag is set. +if the task is to be frozen and makes the task enter __refrigerator(). For user space processes try_to_freeze() is called automatically from the signal-handling code, but the freezable kernel threads need to call it explicitly in suitable places or use the wait_event_freezable() or wait_event_freezable_timeout() macros (defined in include/linux/freezer.h) -that combine interruptible sleep with checking if TIF_FREEZE is set and calling -try_to_freeze(). The main loop of a freezable kernel thread may look like the -following one: +that combine interruptible sleep with checking if the task is to be frozen and +calling try_to_freeze(). The main loop of a freezable kernel thread may look +like the following one: set_freezable(); do { @@ -53,7 +54,7 @@ following one: (from drivers/usb/core/hub.c::hub_thread()). If a freezable kernel thread fails to call try_to_freeze() after the freezer has -set TIF_FREEZE for it, the freezing of tasks will fail and the entire +initiated a freezing operation, the freezing of tasks will fail and the entire hibernation operation will be cancelled. For this reason, freezable kernel threads must call try_to_freeze() somewhere or use one of the wait_event_freezable() and wait_event_freezable_timeout() macros. |