Changes to workqueues (was: Does it make sense to have the hdev workqueue serialized?)

Sorry - I forgot to update my subject line after writing the actual 
email and refining my proposal.

On Fri, 16 Dec 2011, Mat Martineau wrote:

On Thu, 15 Dec 2011, David Herrmann wrote:

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Hi Mat

On Thu, Dec 15, 2011 at 8:25 PM, Mat Martineau [off-list ref] 
wrote:

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Marcel & Andre -


On Wed, 14 Dec 2011, Marcel Holtmann wrote:

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Hi Andre,

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This patch adds to hci_core an infra-structure to scan LE devices.

The LE scan is implemented using a work_struct which is enqueued
on hdev->workqueue. The LE scan work sends commands (Set LE Scan
Parameters and Set LE Scan Enable) to the controller and waits for
its results. If commands were executed successfully a timer is set
to disable the ongoing scanning after some amount of time.

so I rather hold off on these until we get the tasklet removal patches
merged. The mgmt processing will then also be done in process context
and we can just sleep. This should make code like this a lot simpler.

While execution on a workqueue can sleep, it's not a good idea to 
block for a long time like this patch does.  A single-threaded 
workqueue (like the hdev workqueue) will not run the next 
scheduled work until the current work function returns.  If code 
executing in a workqueue suspends execution by using a wait queue, 
like le_scan_workqueue(), then all other pending work is blocked 
until le_scan_workqueue() returns.

Why do we use a single-threaded workqueue anyway? Why don't we switch
to a non-reentrant workqueue? Otherwise, we are just blocking the
whole hdev workqueue because we are too lazy to implement proper
locking between work-structs that depend on each other.

Before 2.6.36, creating a workqueue would create a dedicated thread per 
processor (or just one thread for a single threaded workqueue). I think I've 
seen Marcel comment that we didn't have enough work to justify the extra 
resources for multiple threads.

Since 2.6.36, there are dynamic thread pools for each processor that do not 
depend on the number of workqueues.  Threads are instead allocated based on 
the amount of concurrent work present in the system.  See 
http://lwn.net/Articles/403891/

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It might be better to think of workqueues as having similar restrictions 
to
tasklets, except you can use GFP_KERNEL when allocating and can block 
while
acquiring locks.

That sounds like a lot of work with almost no benefit. If we start the
transition from tasklets to workqueues I think we should do it
properly so we do not require a single-threaded workqueue.

The benefit would be in having no need to keep track of which context 
functions are executing in.  It's been a big headache with the ERTM and AMP 
changes, and there is a bunch of code that could work better in process 
context if it didn't have to also handle calls from tasklets.

That said, after learning more about how workqueues are implemented now, it 
may be worthwhile to change the "use one single-threaded workqueue for 
everything" assumption.  alloc_workqueue() has a max_active parameter, and it 
is possible to have many work items running concurrently.  Some of those 
threads could be suspended, like Andre does in his patch.  Workqueues created 
with the old create_workqueue() or create_singlethread_workqueue() have 
max_active == 1, which enforces serialization on each processor.


So there are two big questions:  Do we want to keep pushing everything on the 
hdev workqueue, since workqueues are not as heavyweight as they used to be? 
And does it make sense to keep our workqueues serialized?


Advantages of serialization:
* An HCI device is serialized by the transport anyway, so it makes sense to 
match that model.
* Ordering is maintained.  The order of incoming HCI events may queue work 
in a particular order and need to assume the work will be executed in that 
order.
* Simplicity.
* No lock contention between multiple workers.

Advantages of not serializing:
* Takes advantage of SMP
* Workers can block without affecting the rest of the queue, enabling 
workers to be long-lived and use state on the thread stack instead of 
complicated lists of pending operations and dynamic allocation.
* We need to have proper locking to deal with user processes anyway, so why 
not allow more concurrency internally.
* Some work can proceed while waiting for locks in other workers.
* Can use WQ_HIGHPRI to keep tx/rx data moving even when workers are waiting 
for locks


I think what's called for is a hybrid approach that serializes where 
necessary, but uses multiple workqueues.  How about this:

* Use the serialized hdev workqueue for rx/tx only.  This could use 
WQ_HIGHPRI to help performance.
* Have a serialized workqueue for each L2CAP channel to handle per-channel 
timeouts.
* Have a global, non-serialized workqueue for stuff like sysfs and mgmt to 
use.


Does that sound workable?

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In getting rid of tasklets, I think we also need to not use timers (which
also execute in softirq context), and use the delayed_work calls instead.
 That way we can assume all net/bluetooth code runs in process context,
except for calls up from the driver layer.

Are there currently any pending patches? I've tried converting the
tasklets to workqueues myself but I always ended up with several
race-conditions. I haven't found a clean and easy way to fix them,
yet. And it's also a quite frustrating work.

Gustavo's working on it, starting from Marcel's patch.  I think it's this 
one: http://www.spinics.net/lists/linux-bluetooth/msg06892.html

--
Mat Martineau
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum