Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap_block->lock

[Dave Chinner]

On Mon, Mar 27, 2023 at 07:22:44PM +0200, Uladzislau Rezki wrote:
> >     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
> OK, i see. I tried to use the fs_mark in different configurations. For
> example:
> 
> <snip>
> time fs_mark -D 10000 -S0 -n 100000 -s 0 -L 32 -d ./scratch/0 -d ./scratch/1 -d ./scratch/2  \
> -d ./scratch/3 -d ./scratch/4 -d ./scratch/5 -d ./scratch/6 -d ./scratch/7 -d ./scratch/8 \
> -d ./scratch/9 -d ./scratch/10 -d ./scratch/11 -d ./scratch/12 -d ./scratch/13 \
> -d ./scratch/14 -d ./scratch/15 -t 64 -F
> <snip>
> 
> But i did not manage to trigger xlog_cil_commit() to fallback to vmalloc
> code. I think i should reduce an amount of memory on my kvm-pc and
> repeat the tests!

Simple way of doing is to use directory blocks that are larger than
page size:

mkfs.xfs -n size=64k ....

We can hit that path in other ways - large attributes will hit it in
the attr buffer allocation path, enabling the new attribute
intent-based logging mode will hit it in the xlog_cil_commit path as
well. IIRC, the above profile comes from the latter case, creating
lots of zero length files with 64kB xattrs attached via fsmark.

-Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx