On Fri, 16 Dec 2011, Gustavo Padovan wrote:
Hi Mat,
* Mat Martineau <mathewm@xxxxxxxxxxxxxx> [2011-12-16 11:20:21 -0800]:
On Thu, 15 Dec 2011, David Herrmann wrote:
Hi Mat
On Thu, Dec 15, 2011 at 8:25 PM, Mat Martineau <mathewm@xxxxxxxxxxxxxx> wrote:
Marcel & Andre -
On Wed, 14 Dec 2011, Marcel Holtmann wrote:
Hi Andre,
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/
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.
I didn't know alloc_workqueue() either, I think its a good idea go for it.
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.
I agree with this one.
* Have a serialized workqueue for each L2CAP channel to handle
per-channel timeouts.
Isn't it too much? maybe one workqueue per l2cap_conn is better.
l2cap_conn does make more sense.
* Have a global, non-serialized workqueue for stuff like sysfs and
mgmt to use.
Some of the workqueue usage we have today might go away after the workqueue
change patches, we can check later if such workqueue will be really needed.
I was thinking it would be useful for jobs like Andre has (a
longer-running thread that blocks at different stages), but one of the
system workqueues (like system_long_wq) would work fine for that. No
need for a special Bluetooth queue.
A long-running thread like this could even be useful for the AMP
manager work that's going on - AMP manager state could be built up in
a thread instead of building an async state machine.
Does that sound workable?
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
As I said on the other e-mail, code is here:
http://git.kernel.org/?p=linux/kernel/git/padovan/bluetooth-testing.git
Comments are welcome!
Thanks for the pointer to bluetooth-testing - I hadn't checked on it
in a while! I will look over your changes.
--
Mat Martineau
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum
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