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

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Date:   Mon, 25 Oct 2021 11:48:41 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     NeilBrown <neilb@...e.de>, Michal Hocko <mhocko@...e.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Michal Hocko <mhocko@...e.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 Fri, Oct 22, 2021 at 09:49:08AM +1100, NeilBrown wrote:
> On Thu, 21 Oct 2021, Uladzislau Rezki wrote:
> > > 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.
> > >
> > 
> > To me the msleep is just more simpler from semantic point of view, i.e.
> > it is as straight forward as it can be. In case of interruptable/uninteraptable
> > sleep it can be more confusing for people.
> 
> I agree that msleep() is more simple.  I think adding the
> jiffies_to_msec() substantially reduces that simplicity.
> 
> > 
> > When it comes to rounding and possibility to sleep more than 1 tick, it
> > really does not matter here, we do not need to guarantee exact sleeping
> > time.
> > 
> > Therefore i proposed to switch to the msleep().
> 
> If, as you say, the precision doesn't matter that much, then maybe
>    msleep(0)
> which would sleep to the start of the next jiffy.  Does that look a bit
> weird?  If so, the msleep(1) would be ok.
> 
Agree, msleep(1) looks much better rather then converting 1 jiffy to
milliseconds. Result should be the same.

> However now that I've thought about some more, I'd much prefer we
> introduce something like
>     memalloc_retry_wait();
> 
> and use that everywhere that a memory allocation is retried.
> I'm not convinced that we need to wait at all - at least, not when
> __GFP_DIRECT_RECLAIM is used, as in that case alloc_page will either
>   - succeed
>   - make some progress a reclaiming or
>   - sleep
> 
> However I'm not 100% certain, and the behaviour might change in the
> future.  So having one place (the definition of memalloc_retry_wait())
> where we can change the sleeping behaviour if the alloc_page behavour
> changes, would be ideal.  Maybe memalloc_retry_wait() could take a
> gfpflags arg.
> 
At sleeping is required for __get_vm_area_node() because in case of lack
of vmap space it will end up in tight loop without sleeping what is
really bad.

Thanks!

--
Vlad Rezki