linux-kernel - Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup

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Message-Id: <20200109142758.659c1545cb8df2d05f299a4a@linux-foundation.org>
Date:   Thu, 9 Jan 2020 14:27:58 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Scott Cheloha <cheloha@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        nathanl@...ux.ibm.com, ricklind@...ux.vnet.ibm.com,
        mhocko@...e.com, Scott Cheloha <cheloha@...ux.ibm.com>
Subject: Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to
 accelerate lookup

On Thu, 9 Jan 2020 23:17:09 +0100 David Hildenbrand <david@...hat.com> wrote:

> 
> 
> > Am 09.01.2020 um 23:00 schrieb Andrew Morton <akpm@...ux-foundation.org>:
> > 
> > On Thu,  9 Jan 2020 15:25:16 -0600 Scott Cheloha <cheloha@...ux.vnet.ibm.com> wrote:
> > 
> >> Searching for a particular memory block by id is an O(n) operation
> >> because each memory block's underlying device is kept in an unsorted
> >> linked list on the subsystem bus.
> >> 
> >> We can cut the lookup cost to O(log n) if we cache the memory blocks in
> >> a radix tree.  With a radix tree cache in place both memory subsystem
> >> initialization and memory hotplug run palpably faster on systems with a
> >> large number of memory blocks.
> >> 
> >> ...
> >> 
> >> @@ -56,6 +57,13 @@ static struct bus_type memory_subsys = {
> >>    .offline = memory_subsys_offline,
> >> };
> >> 
> >> +/*
> >> + * Memory blocks are cached in a local radix tree to avoid
> >> + * a costly linear search for the corresponding device on
> >> + * the subsystem bus.
> >> + */
> >> +static RADIX_TREE(memory_blocks, GFP_KERNEL);
> > 
> > What protects this tree from racy accesses?
> 
> I think the device hotplug lock currently (except during boot where no races can happen).
> 

So this?

--- a/drivers/base/memory.c~drivers-base-memoryc-cache-blocks-in-radix-tree-to-accelerate-lookup-fix
+++ a/drivers/base/memory.c
@@ -61,6 +61,9 @@ static struct bus_type memory_subsys = {
  * Memory blocks are cached in a local radix tree to avoid
  * a costly linear search for the corresponding device on
  * the subsystem bus.
+ *
+ * Protected by mem_hotplug_lock in mem_hotplug_begin(), and by the guaranteed
+ * single-threadness at boot time.
  */
 static RADIX_TREE(memory_blocks, GFP_KERNEL);
 

But are we sure this is all true?