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

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Date:   Tue, 27 Feb 2018 05:06:43 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "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, 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!

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.

My initial proposal would be to reuse the vmap_area structure and store
the freed ones in a second rb_tree sorted by the size (ie va_end - va_start).
When freeing, we might need to merge forwards and backwards.  Allocating
would be a matter of finding an area preferably of the exact right size;
otherwise split a larger free area into a free area and an allocated area
(there's a lot of literature on how exactly to choose which larger area
to split; memory allocators are pretty well-studied).