linux-kernel - Re: [RFC 2/3] mm/vmalloc: add support for __GFP

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Date:   Thu, 21 Oct 2021 12:27:28 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     NeilBrown <neilb@...e.de>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Dave Chinner <david@...morbit.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>
Subject: Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL

On Thu 21-10-21 21:13:35, Neil Brown wrote:
> On Thu, 21 Oct 2021, Uladzislau Rezki wrote:
> > On Wed, Oct 20, 2021 at 05:00:28PM +0200, Uladzislau Rezki wrote:
> > > >
> > > > On Wed 20-10-21 16:29:14, Uladzislau Rezki wrote:
> > > > > On Wed, Oct 20, 2021 at 4:06 PM Michal Hocko <mhocko@...e.com> wrote:
> > > > [...]
> > > > > > As I've said I am OK with either of the two. Do you or anybody have any
> > > > > > preference? Without any explicit event to wake up for neither of the two
> > > > > > is more than just an optimistic retry.
> > > > > >
> > > > > From power perspective it is better to have a delay, so i tend to say
> > > > > that delay is better.
> > > >
> > > > I am a terrible random number generator. Can you give me a number
> > > > please?
> > > >
> > > Well, we can start from one jiffy so it is one timer tick: schedule_timeout(1)
> > > 
> > A small nit, it is better to replace it by the simple msleep() call: msleep(jiffies_to_msecs(1));
> 
> I disagree.  I think schedule_timeout_uninterruptible(1) is the best
> wait to sleep for 1 ticl
> 
> msleep() contains
>   timeout = msecs_to_jiffies(msecs) + 1;
> and both jiffies_to_msecs and msecs_to_jiffies might round up too.
> So you will sleep for at least twice as long as you asked for, possible
> more.

That was my thinking as well. Not to mention jiffies_to_msecs just to do
msecs_to_jiffies right after which seems like a pointless wasting of
cpu cycle. But maybe I was missing some other reasons why msleep would
be superior.
-- 
Michal Hocko
SUSE Labs