Re: [PATCH] mm/zswap: Improve with alloc_workqueue() call

[Nhat Pham <nphamcs@gmail.com>]

On Wed, Dec 13, 2023 at 5:20 AM Ronald Monthero
<debug.penguin32@gmail.com> wrote:
>
> Hi Nhat,
> Thanks for checking.
> On Tue, Dec 12, 2023 at 12:16 AM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Sun, Dec 10, 2023 at 9:31 PM Ronald Monthero
> > <debug.penguin32@gmail.com> wrote:
> > >
> > > Use alloc_workqueue() to create and set finer
> > > work item attributes instead of create_workqueue()
> > > which is to be deprecated.
> > >
> > > Signed-off-by: Ronald Monthero <debug.penguin32@gmail.com>
> > > ---
> > >  mm/zswap.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > index 74411dfdad92..64dbe3e944a2 100644
> > > --- a/mm/zswap.c
> > > +++ b/mm/zswap.c
> > > @@ -1620,7 +1620,8 @@ static int zswap_setup(void)
> > >                 zswap_enabled = false;
> > >         }
> > >
> > > -       shrink_wq = create_workqueue("zswap-shrink");
> > > +       shrink_wq = alloc_workqueue("zswap-shrink",
> > > +                       WQ_UNBOUND|WQ_MEM_RECLAIM, 0);
> >
> > Hmmm this changes the current behavior a bit right? create_workqueue()
> > is currently defined as:
> >
> > alloc_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, 1, (name))
>
> create_workqueue is deprecated and it's observed that most of the
> subsystems have changed to using alloc_workqueue. So it's a small
> minority of few remnant instances in the kernel and some drivers still
> using create_workqueue. With the create_workqueue defined as is , it
> hardcodes the workqueue  flags to be per cpu and unbound in nature and
> not giving the flexibility of using any other wait queue
> flags/attributes. ( WQ_CPU_INTENSIVE, WQ_HIGHPRI, WQ_RESCUER,
> WQ_FREEZEABLE, WQ_UNBOUND) . Hence most of the subsystems and drivers
> use the alloc_workqueue( ) api.
> #define create_workqueue(name) \
> alloc_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, 1, (name))
>
> > I think this should be noted in the changelog, at the very least, even
> > if it is fine. We should be as explicit as possible about behavior
> > changes.
> >
> imo, it's sort of known and consistently changed for quite some time already.
> https://lists.openwall.net/linux-kernel/2016/06/07/1086
> https://lists.openwall.net/linux-kernel/2011/01/03/124
> https://lwn.net/Articles/403891/   => quoted: "The long-term plan, it
> seems, is to convert all create_workqueue() users over to an
> appropriate alloc_workqueue() call; eventually create_workqueue() will
> be removed"
>
> glad to take some suggestions , thoughts ?
>
> BR,
> ronald

I should have been clearer. I'm not against the change per-se - I
agree that we should replace create_workqueue() with
alloc_workqueue(). What I meant was, IIUC, there are two behavioral
changes with this new workqueue creation:

a) We're replacing a bounded workqueue (which as you noted, is fixed
by create_workqueue()) with an unbounded one (WQ_UNBOUND). This seems
fine to me - I doubt locality buys us much here.

b) create_workqueue() limits the number of concurrent per-cpu
execution contexts at 1 (i.e only one single global reclaimer),
whereas after this patch this is set to the default value. This seems
fine to me too - I don't remember us taking advantage of the previous
concurrency limitation. Also, in practice, the task_struct is
one-to-one with the zswap_pool's anyway, and most of the time, there
is just a single pool being used. (But it begs the question - what's
the point of using 0 instead of 1 here?)

Both seem fine (to me anyway - other reviewers feel free to take a
look and fact-check everything). I just feel like this should be
explicitly noted in the changelog, IMHO, in case we are mistaken and
need to revisit this :) Either way, not a NACK from me.