Re: [RFC PATCH v1 1/4] Introducing qpw_lock() and per-cpu queue & flush work

[Leonardo Bras <leobras@redhat.com>]

On Wed, Sep 04, 2024 at 08:08:12PM -0400, Waiman Long wrote:
> On 9/4/24 17:39, Waiman Long wrote:
> > On 6/21/24 23:58, Leonardo Bras wrote:
> > > Some places in the kernel implement a parallel programming strategy
> > > consisting on local_locks() for most of the work, and some rare remote
> > > operations are scheduled on target cpu. This keeps cache bouncing
> > > low since
> > > cacheline tends to be mostly local, and avoids the cost of locks in
> > > non-RT
> > > kernels, even though the very few remote operations will be
> > > expensive due
> > > to scheduling overhead.
> > > 
> > > On the other hand, for RT workloads this can represent a problem:
> > > getting
> > > an important workload scheduled out to deal with some unrelated task is
> > > sure to introduce unexpected deadline misses.
> > > 
> > > It's interesting, though, that local_lock()s in RT kernels become
> > > spinlock(). We can make use of those to avoid scheduling work on a
> > > remote
> > > cpu by directly updating another cpu's per_cpu structure, while holding
> > > it's spinlock().
> > > 
> > > In order to do that, it's necessary to introduce a new set of
> > > functions to
> > > make it possible to get another cpu's per-cpu "local" lock
> > > (qpw_{un,}lock*)
> > > and also the corresponding queue_percpu_work_on() and
> > > flush_percpu_work()
> > > helpers to run the remote work.
> > > 
> > > On non-RT kernels, no changes are expected, as every one of the
> > > introduced
> > > helpers work the exactly same as the current implementation:
> > > qpw_{un,}lock*()        ->  local_{un,}lock*() (ignores cpu parameter)
> > > queue_percpu_work_on()  ->  queue_work_on()
> > > flush_percpu_work()     ->  flush_work()
> > > 
> > > For RT kernels, though, qpw_{un,}lock*() will use the extra cpu
> > > parameter
> > > to select the correct per-cpu structure to work on, and acquire the
> > > spinlock for that cpu.
> > > 
> > > queue_percpu_work_on() will just call the requested function in the
> > > current
> > > cpu, which will operate in another cpu's per-cpu object. Since the
> > > local_locks() become spinlock()s in PREEMPT_RT, we are safe doing that.
> > > 
> > > flush_percpu_work() then becomes a no-op since no work is actually
> > > scheduled on a remote cpu.
> > > 
> > > Some minimal code rework is needed in order to make this mechanism work:
> > > The calls for local_{un,}lock*() on the functions that are currently
> > > scheduled on remote cpus need to be replaced by qpw_{un,}lock_n*(),
> > > so in
> > > RT kernels they can reference a different cpu. It's also necessary
> > > to use a
> > > qpw_struct instead of a work_struct, but it just contains a work struct
> > > and, in PREEMPT_RT, the target cpu.
> > > 
> > > This should have almost no impact on non-RT kernels: few this_cpu_ptr()
> > > will become per_cpu_ptr(,smp_processor_id()).
> > > 
> > > On RT kernels, this should improve performance and reduce latency by
> > > removing scheduling noise.
> > > 
> > > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > > ---
> > >   include/linux/qpw.h | 88 +++++++++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 88 insertions(+)
> > >   create mode 100644 include/linux/qpw.h
> > > 
> > > diff --git a/include/linux/qpw.h b/include/linux/qpw.h
> > > new file mode 100644
> > > index 000000000000..ea2686a01e5e
> > > --- /dev/null
> > > +++ b/include/linux/qpw.h
> > > @@ -0,0 +1,88 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +#ifndef _LINUX_QPW_H
> > > +#define _LINUX_QPW_H
> 
> I would suggest adding a comment with a brief description of what
> qpw_lock/unlock() are for and their use cases. The "qpw" prefix itself isn't
> intuitive enough for a casual reader to understand what they are for.

Agree, I am also open to discuss a more intuitive naming for these.

> 
> Cheers,
> Longman
> 

Thanks!
Leo