Re: [PATCH] proc: Avoid costly high-order page allocations when reading proc files

Dave Chinner
2025-04-02T11:32:22Z

On Wed, Apr 02, 2025 at 04:42:06PM +0800, Yafang Shao wrote:
> On Wed, Apr 2, 2025 at 12:15 PM Harry Yoo <harry.yoo@oracle.com> wrote:
> >
> > On Tue, Apr 01, 2025 at 07:01:04AM -0700, Kees Cook wrote:
> > >
> > >
> > > On April 1, 2025 12:30:46 AM PDT, Yafang Shao <laoar.shao@gmail.com> wrote:
> > > >While investigating a kcompactd 100% CPU utilization issue in production, I
> > > >observed frequent costly high-order (order-6) page allocations triggered by
> > > >proc file reads from monitoring tools. This can be reproduced with a simple
> > > >test case:
> > > >
> > > > fd = open(PROC_FILE, O_RDONLY);
> > > > size = read(fd, buff, 256KB);
> > > > close(fd);
> > > >
> > > >Although we should modify the monitoring tools to use smaller buffer sizes,
> > > >we should also enhance the kernel to prevent these expensive high-order
> > > >allocations.
> > > >
> > > >Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > > >Cc: Josef Bacik <josef@toxicpanda.com>
> > > >---
> > > > fs/proc/proc_sysctl.c | 10 +++++++++-
> > > > 1 file changed, 9 insertions(+), 1 deletion(-)
> > > >
> > > >diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > > >index cc9d74a06ff0..c53ba733bda5 100644
> > > >--- a/fs/proc/proc_sysctl.c
> > > >+++ b/fs/proc/proc_sysctl.c
> > > >@@ -581,7 +581,15 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, struct iov_iter *iter,
> > > > error = -ENOMEM;
> > > > if (count >= KMALLOC_MAX_SIZE)
> > > > goto out;
> > > >- kbuf = kvzalloc(count + 1, GFP_KERNEL);
> > > >+
> > > >+ /*
> > > >+ * Use vmalloc if the count is too large to avoid costly high-order page
> > > >+ * allocations.
> > > >+ */
> > > >+ if (count < (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
> > > >+ kbuf = kvzalloc(count + 1, GFP_KERNEL);
> > >
> > > Why not move this check into kvmalloc family?
> >
> > Hmm should this check really be in kvmalloc family?
>
> Modifying the existing kvmalloc functions risks performance regressions.
> Could we instead introduce a new variant like vkmalloc() (favoring
> vmalloc over kmalloc) or kvmalloc_costless()?

We should fix kvmalloc() instead of continuing to force
subsystems to work around the limitations of kvmalloc().

Have a look at xlog_kvmalloc() in XFS. It implements a basic
fast-fail, no retry high order kmalloc before it falls back to
vmalloc by turning off direct reclaim for the kmalloc() call.
Hence if the there isn't a high-order page on the free lists ready
to allocate, it falls back to vmalloc() immediately.

For XFS, using xlog_kvmalloc() reduced the high-order per-allocation
overhead by around 80% when compared to a standard kvmalloc()
call. Numbers and profiles were documented in the commit message
(reproduced in whole below)...

> > I don't think users would expect kvmalloc() to implictly decide on using
> > vmalloc() without trying kmalloc() first, just because it's a high-order
> > allocation.

Right, but users expect kvmalloc() to use the most efficient
allocation paths available to it.

In this case, vmalloc() is faster and more reliable than
direct reclaim w/ compaction. Hence vmalloc() should really be the
primary fallback path when high-order pages are not immediately
available to kmalloc() when called from kvmalloc()...

-Dave.
--
Dave Chinner
david@fromorbit.com

commit 8dc9384b7d75012856b02ff44c37566a55fc2abf
Author: Dave Chinner <dchinner@redhat.com>
Date: Tue Jan 4 17:22:18 2022 -0800

xfs: reduce kvmalloc overhead for CIL shadow buffers

Oh, let me count the ways that the kvmalloc API sucks dog eggs.

The problem is when we are logging lots of large objects, we hit
kvmalloc really damn hard with costly order allocations, and
behaviour utterly sucks:

- 49.73% xlog_cil_commit
- 31.62% kvmalloc_node
- 29.96% __kmalloc_node
- 29.38% kmalloc_large_node
- 29.33% __alloc_pages
- 24.33% __alloc_pages_slowpath.constprop.0
- 18.35% __alloc_pages_direct_compact
- 17.39% try_to_compact_pages
- compact_zone_order
- 15.26% compact_zone
5.29% __pageblock_pfn_to_page
3.71% PageHuge
- 1.44% isolate_migratepages_block
0.71% set_pfnblock_flags_mask
1.11% get_pfnblock_flags_mask
- 0.81% get_page_from_freelist
- 0.59% _raw_spin_lock_irqsave
- do_raw_spin_lock
__pv_queued_spin_lock_slowpath
- 3.24% try_to_free_pages
- 3.14% shrink_node
- 2.94% shrink_slab.constprop.0
- 0.89% super_cache_count
- 0.66% xfs_fs_nr_cached_objects
- 0.65% xfs_reclaim_inodes_count
0.55% xfs_perag_get_tag
0.58% kfree_rcu_shrink_count
- 2.09% get_page_from_freelist
- 1.03% _raw_spin_lock_irqsave
- do_raw_spin_lock
__pv_queued_spin_lock_slowpath
- 4.88% get_page_from_freelist
- 3.66% _raw_spin_lock_irqsave
- do_raw_spin_lock
__pv_queued_spin_lock_slowpath
- 1.63% __vmalloc_node
- __vmalloc_node_range
- 1.10% __alloc_pages_bulk
- 0.93% __alloc_pages
- 0.92% get_page_from_freelist
- 0.89% rmqueue_bulk
- 0.69% _raw_spin_lock
- do_raw_spin_lock
__pv_queued_spin_lock_slowpath
13.73% memcpy_erms
- 2.22% kvfree

On this workload, that's almost a dozen CPUs all trying to compact
and reclaim memory inside kvmalloc_node at the same time. Yet it is
regularly falling back to vmalloc despite all that compaction, page
and shrinker reclaim that direct reclaim is doing. Copying all the
metadata is taking far less CPU time than allocating the storage!

Direct reclaim should be considered extremely harmful.

This is a high frequency, high throughput, CPU usage and latency
sensitive allocation. We've got memory there, and we're using
kvmalloc to allow memory allocation to avoid doing lots of work to
try to do contiguous allocations.

Except it still does *lots of costly work* that is unnecessary.

Worse: the only way to avoid the slowpath page allocation trying to
do compaction on costly allocations is to turn off direct reclaim
(i.e. remove __GFP_RECLAIM_DIRECT from the gfp flags).

Unfortunately, the stupid kvmalloc API then says "oh, this isn't a
GFP_KERNEL allocation context, so you only get kmalloc!". This
cuts off the vmalloc fallback, and this leads to almost instant OOM
problems which ends up in filesystems deadlocks, shutdowns and/or
kernel crashes.

I want some basic kvmalloc behaviour:

- kmalloc for a contiguous range with fail fast semantics - no
compaction direct reclaim if the allocation enters the slow path.
- run normal vmalloc (i.e. GFP_KERNEL) if kmalloc fails

The really, really stupid part about this is these kvmalloc() calls
are run under memalloc_nofs task context, so all the allocations are
always reduced to GFP_NOFS regardless of the fact that kvmalloc
requires GFP_KERNEL to be passed in. IOWs, we're already telling
kvmalloc to behave differently to the gfp flags we pass in, but it
still won't allow vmalloc to be run with anything other than
GFP_KERNEL.

So, this patch open codes the kvmalloc() in the commit path to have
the above described behaviour. The result is we more than halve the
CPU time spend doing kvmalloc() in this path and transaction commits
with 64kB objects in them more than doubles. i.e. we get ~5x
reduction in CPU usage per costly-sized kvmalloc() invocation and
the profile looks like this:

- 37.60% xlog_cil_commit
16.01% memcpy_erms
- 8.45% __kmalloc
- 8.04% kmalloc_order_trace
- 8.03% kmalloc_order
- 7.93% alloc_pages
- 7.90% __alloc_pages
- 4.05% __alloc_pages_slowpath.constprop.0
- 2.18% get_page_from_freelist
- 1.77% wake_all_kswapds
....
- __wake_up_common_lock
- 0.94% _raw_spin_lock_irqsave
- 3.72% get_page_from_freelist
- 2.43% _raw_spin_lock_irqsave
- 5.72% vmalloc
- 5.72% __vmalloc_node_range
- 4.81% __get_vm_area_node.constprop.0
- 3.26% alloc_vmap_area
- 2.52% _raw_spin_lock
- 1.46% _raw_spin_lock
0.56% __alloc_pages_bulk
- 4.66% kvfree
- 3.25% vfree
- __vfree
- 3.23% __vunmap
- 1.95% remove_vm_area
- 1.06% free_vmap_area_noflush
- 0.82% _raw_spin_lock
- 0.68% _raw_spin_lock
- 0.92% _raw_spin_lock
- 1.40% kfree
- 1.36% __free_pages
- 1.35% __free_pages_ok
- 1.02% _raw_spin_lock_irqsave

It's worth noting that over 50% of the CPU time spent allocating
these shadow buffers is now spent on spinlocks. So the shadow buffer
allocation overhead is greatly reduced by getting rid of direct
reclaim from kmalloc, and could probably be made even less costly if
vmalloc() didn't use global spinlocks to protect it's structures.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>