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

Uladzislau Rezki
2023-03-28T12:41:06Z

On Tue, Mar 28, 2023 at 01:53:27PM +1100, Dave Chinner wrote:
> 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.
>
Good. Thank you that is useful.

--
Uladzislau Rezki