From: Peter Zijlstra <peterz@infradead.org>
To: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Shakeel Butt <shakeelb@google.com>,
Oscar Salvador <osalvador@suse.de>,
David Hildenbrand <david@redhat.com>,
Muchun Song <songmuchun@bytedance.com>,
David Rientjes <rientjes@google.com>,
Miaohe Lin <linmiaohe@huawei.com>,
Matthew Wilcox <willy@infradead.org>,
HORIGUCHI NAOYA <naoya.horiguchi@nec.com>,
"Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
Waiman Long <longman@redhat.com>, Peter Xu <peterx@redhat.com>,
Mina Almasry <almasrymina@google.com>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 2/8] hugetlb: recompute min_count when dropping hugetlb_lock
Date: Tue, 23 Mar 2021 09:01:02 +0100 [thread overview]
Message-ID: <YFmgPkTzZY6Ocj6X@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YFmd3d5B2VT4GkiG@dhcp22.suse.cz>
On Tue, Mar 23, 2021 at 08:50:53AM +0100, Michal Hocko wrote:
> > >> +static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
> > >> +{
> > >> + unsigned long min_count;
> > >> +
> > >> + min_count = h->resv_huge_pages + h->nr_huge_pages - h->free_huge_pages;
> > >> + return max(count, min_count);
> > >
> > > Just out of curiousity, is compiler allowed to inline this piece of code
> > > and then cache the value? In other words do we need to make these
> > > READ_ONCE or otherwise enforce the no-caching behavior?
> >
> > I honestly do not know if the compiler is allowed to do that. The
> > assembly code generated by my compiler does not cache the value, but
> > that does not guarantee anything. I can add READ_ONCE to make the
> > function look something like:
> >
> > static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
> > {
> > unsigned long min_count;
> >
> > min_count = READ_ONCE(h->resv_huge_pages) + READ_ONCE(h->nr_huge_pages)
> > - READ_ONCE(h->free_huge_pages);
> > return max(count, min_count);
> > }
>
> Maybe just forcing to never inline the function should be sufficient.
> This is not a hot path to micro optimize for no function call. But there
> are much more qualified people on the CC list on this matter who could
> clarify. Peter?
I'm not sure I understand the code right. But inline or not doesn't
matter, LTO completely ruins that game. Just like if it was a static
function, then the compiler is free to inline it, even if the function
lacks an inline attribute.
Basically, without READ_ONCE() the compiler is allowed to entirely elide
the load (and use a previous load), or to duplicate the load and do it
again later (reaching a different result).
Similarly, the compiler is allowed to byte-wise load the variable in any
random order and re-assemble.
If any of that is a problem, you have to use READ_ONCE().
next prev parent reply other threads:[~2021-03-23 8:01 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-19 22:42 [RFC PATCH 0/8] make hugetlb put_page safe for all calling contexts Mike Kravetz
2021-03-19 22:42 ` [RFC PATCH 1/8] hugetlb: add per-hstate mutex to synchronize user adjustments Mike Kravetz
2021-03-22 13:59 ` Michal Hocko
2021-03-22 16:57 ` Mike Kravetz
2021-03-23 7:48 ` Michal Hocko
2021-03-19 22:42 ` [RFC PATCH 2/8] hugetlb: recompute min_count when dropping hugetlb_lock Mike Kravetz
2021-03-22 14:07 ` Michal Hocko
2021-03-22 23:07 ` Mike Kravetz
2021-03-23 7:50 ` Michal Hocko
2021-03-23 8:01 ` Peter Zijlstra [this message]
2021-03-23 8:14 ` Michal Hocko
2021-03-23 23:18 ` Mike Kravetz
2021-03-24 8:36 ` Michal Hocko
2021-03-24 16:43 ` Mike Kravetz
2021-03-19 22:42 ` [RFC PATCH 3/8] hugetlb: create remove_hugetlb_page() to separate functionality Mike Kravetz
2021-03-22 14:15 ` Michal Hocko
2021-03-22 17:01 ` Mike Kravetz
2021-03-19 22:42 ` [RFC PATCH 4/8] hugetlb: call update_and_free_page without hugetlb_lock Mike Kravetz
2021-03-22 14:19 ` Michal Hocko
2021-03-19 22:42 ` [RFC PATCH 5/8] hugetlb: change free_pool_huge_page to remove_pool_huge_page Mike Kravetz
2021-03-22 14:31 ` Michal Hocko
2021-03-22 23:28 ` Mike Kravetz
2021-03-23 7:57 ` Michal Hocko
2021-03-24 1:03 ` Mike Kravetz
2021-03-24 8:40 ` Michal Hocko
2021-03-24 16:38 ` Mike Kravetz
2021-03-24 16:50 ` Michal Hocko
2021-03-19 22:42 ` [RFC PATCH 6/8] hugetlb: make free_huge_page irq safe Mike Kravetz
2021-03-21 19:55 ` Mike Kravetz
2021-03-22 13:36 ` [hugetlb] cd190f60f9: BUG:sleeping_function_called_from_invalid_context_at_mm/hugetlb.c kernel test robot
2021-03-22 14:35 ` [RFC PATCH 6/8] hugetlb: make free_huge_page irq safe Michal Hocko
2021-03-19 22:42 ` [RFC PATCH 7/8] hugetlb: add update_and_free_page_no_sleep for irq context Mike Kravetz
2021-03-20 1:18 ` Hillf Danton
2021-03-25 0:26 ` Mike Kravetz
2021-03-22 8:41 ` Peter Zijlstra
2021-03-22 17:42 ` Mike Kravetz
2021-03-22 18:10 ` Roman Gushchin
2021-03-23 18:51 ` Mike Kravetz
2021-03-23 19:07 ` Roman Gushchin
2021-03-24 8:43 ` Michal Hocko
2021-03-24 16:53 ` Mike Kravetz
2021-03-22 20:43 ` Peter Zijlstra
2021-03-22 14:42 ` Michal Hocko
2021-03-22 14:46 ` Michal Hocko
2021-03-19 22:42 ` [RFC PATCH 8/8] hugetlb: track hugetlb pages allocated via cma_alloc Mike Kravetz
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