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

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Message-Id: <20180302153452.748892bd70bb23b9cee23691@linux-foundation.org>
Date:   Fri, 2 Mar 2018 15:34:52 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "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 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?