From: John Hubbard <jhubbard@nvidia.com>
To: Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>
Cc: <linux-fsdevel@vger.kernel.org>, <linux-mm@kvack.org>
Subject: Re: [PATCH 8/8] xarray: Don't clear marks in xas_store()
Date: Wed, 5 Feb 2020 14:19:34 -0800 [thread overview]
Message-ID: <8ea2682b-7240-dca3-b123-2df7d0c994ba@nvidia.com> (raw)
In-Reply-To: <20200204142514.15826-9-jack@suse.cz>
On 2/4/20 6:25 AM, Jan Kara wrote:
> When storing NULL in xarray, xas_store() has been clearing all marks
> because it could otherwise confuse xas_for_each_marked(). That is
> however no longer true and no current user relies on this behavior.
However, let's not forget that the API was also documented to behave
in this way--it's not an accidental detail. Below...
> Furthermore it seems as a cleaner API to not do clearing behind caller's
> back in case we store NULL.
>
> This provides a nice boost to truncate numbers due to saving unnecessary
> tag initialization when clearing shadow entries. Sample benchmark
> showing time to truncate 128 files 1GB each on machine with 64GB of RAM
> (so about half of entries are shadow entries):
>
> AVG STDDEV
> Vanilla 4.825s 0.036
> Patched 4.516s 0.014
>
> So we can see about 6% reduction in overall truncate time.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> lib/xarray.c | 9 ---------
> 1 file changed, 9 deletions(-)
>
> diff --git a/lib/xarray.c b/lib/xarray.c
> index 4e32497c51bd..f165e83652f1 100644
> --- a/lib/xarray.c
> +++ b/lib/xarray.c
> @@ -799,17 +799,8 @@ void *xas_store(struct xa_state *xas, void *entry)
> if (xas->xa_sibs)
> xas_squash_marks(xas);
> }
> - if (!entry)
> - xas_init_marks(xas);
>
> for (;;) {
> - /*
> - * Must clear the marks before setting the entry to NULL,
> - * otherwise xas_for_each_marked may find a NULL entry and
> - * stop early. rcu_assign_pointer contains a release barrier
> - * so the mark clearing will appear to happen before the
> - * entry is set to NULL.
> - */
So if we do this, I think we'd also want something like this (probably with
better wording, this is just a first draft):
diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
index 640934b6f7b4..8adeaa8c012e 100644
--- a/Documentation/core-api/xarray.rst
+++ b/Documentation/core-api/xarray.rst
@@ -66,10 +66,11 @@ pointer at every index.
You can then set entries using xa_store() and get entries
using xa_load(). xa_store will overwrite any entry with the
new entry and return the previous entry stored at that index. You can
-use xa_erase() instead of calling xa_store() with a
+use xa_erase() plus xas_init_marks(), instead of calling xa_store() with a
``NULL`` entry. There is no difference between an entry that has never
-been stored to, one that has been erased and one that has most recently
-had ``NULL`` stored to it.
+been stored to and one that has been erased. Those, in turn, are the same
+as an entry that has had ``NULL`` stored to it and also had its marks
+erased via xas_init_marks().
You can conditionally replace an entry at an index by using
xa_cmpxchg(). Like cmpxchg(), it will only succeed if
> rcu_assign_pointer(*slot, entry);
> if (xa_is_node(next) && (!node || node->shift))
> xas_free_nodes(xas, xa_to_node(next));
>
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2020-02-05 22:19 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-04 14:25 [PATCH 0/8] mm: Speedup page cache truncation Jan Kara
2020-02-04 14:25 ` [PATCH 1/8] xarray: Fix premature termination of xas_for_each_marked() Jan Kara
2020-03-12 21:45 ` Matthew Wilcox
2020-03-16 9:16 ` Jan Kara
2020-02-04 14:25 ` [PATCH 2/8] xarray: Provide xas_erase() helper Jan Kara
2020-03-14 19:54 ` Matthew Wilcox
2020-03-16 9:21 ` Jan Kara
2020-03-17 15:28 ` Matthew Wilcox
2020-04-15 16:12 ` Jan Kara
2020-02-04 14:25 ` [PATCH 3/8] xarray: Explicitely set XA_FREE_MARK in __xa_cmpxchg() Jan Kara
2020-02-05 18:45 ` Jason Gunthorpe
2020-02-06 8:03 ` Jan Kara
2020-03-17 15:12 ` Matthew Wilcox
2020-02-04 14:25 ` [PATCH 4/8] mm: Use xas_erase() in page_cache_delete_batch() Jan Kara
2020-02-04 14:25 ` [PATCH 5/8] dax: Use xas_erase() in __dax_invalidate_entry() Jan Kara
2020-02-04 14:25 ` [PATCH 6/8] idr: Use xas_erase() in ida_destroy() Jan Kara
2020-02-04 14:25 ` [PATCH 7/8] mm: Use xas_erase() in collapse_file() Jan Kara
2020-02-04 14:25 ` [PATCH 8/8] xarray: Don't clear marks in xas_store() Jan Kara
2020-02-05 18:43 ` Jason Gunthorpe
2020-02-05 21:59 ` Matthew Wilcox
2020-02-06 13:49 ` Jason Gunthorpe
2020-02-06 14:36 ` Jan Kara
2020-02-06 14:49 ` Jason Gunthorpe
2020-02-05 22:19 ` John Hubbard [this message]
2020-02-06 2:21 ` Matthew Wilcox
2020-02-06 3:48 ` John Hubbard
2020-02-06 4:28 ` Matthew Wilcox
2020-02-06 4:37 ` John Hubbard
2020-02-06 8:36 ` Jan Kara
2020-02-06 8:04 ` Jan Kara
2020-02-06 14:40 ` [PATCH 0/8] mm: Speedup page cache truncation David Sterba
2020-02-18 9:25 ` Jan Kara
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