linux-kernel - Re: [PATCH] mm/zswap: Improve with alloc

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Date:   Wed, 13 Dec 2023 23:20:04 +1000
From:   Ronald Monthero <debug.penguin32@...il.com>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     sjenning@...hat.com, akpm@...ux-foundation.org,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/zswap: Improve with alloc_workqueue() call

Hi Nhat,
Thanks for checking.
On Tue, Dec 12, 2023 at 12:16 AM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Sun, Dec 10, 2023 at 9:31 PM Ronald Monthero
> <debug.penguin32@...il.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@...il.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