linux-kernel - Re: [RFC v1] mm: add the preempt check into alloc_vmap

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Date:   Sat, 3 Mar 2018 22:18:05 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC v1] mm: add the preempt check into alloc_vmap_area()

On Fri, Mar 02, 2018 at 03:34:52PM -0800, Andrew Morton wrote:
> On Tue, 27 Feb 2018 05:06:43 -0800 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > On Tue, Feb 27, 2018 at 11:22:59AM +0100, Uladzislau Rezki (Sony) wrote:
> > > During finding a suitable hole in the vmap_area_list
> > > there is an explicit rescheduling check for latency reduction.
> > > We do it, since there are workloads which are sensitive for
> > > long (more than 1 millisecond) preemption off scenario.
> > 
> > I understand your problem, but this is a horrid solution.  If it takes
> > us a millisecond to find a suitable chunk of free address space, something
> > is terribly wrong.  On a 3GHz CPU, that's 3 million clock ticks!
> 
> Yup.
> 
> > I think our real problem is that we have no data structure that stores
> > free VA space.  We have the vmap_area which stores allocated space, but no
> > data structure to store free space.
> 
> I wonder if we can reuse free_vmap_cache as a quick fix: if
> need_resched(), point free_vmap_cache at the current rb_node, drop the
> lock, cond_resched, goto retry?
> 
It sounds like we can. But there is a concern if that potentially can
introduce a degrade of search time due to changing a starting point
for our search.

--
Vlad Rezki