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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?
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