linux-kernel - Re: [PATCH v8 0/9] rwsem performance optimizations

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Date:	Wed, 09 Oct 2013 22:03:57 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Waiman Long <Waiman.Long@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v8 0/9] rwsem performance optimizations

On Wed, 2013-10-09 at 20:14 -0700, Linus Torvalds wrote:
> On Wed, Oct 9, 2013 at 12:28 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > The workload that I got the report from was a virus scanner, it would
> > spawn nr_cpus threads and {mmap file, scan content, munmap} through your
> > filesystem.
> 
> So I suspect we could make the mmap_sem write area *much* smaller for
> the normal cases.
> 
> Look at do_mmap_pgoff(), for example: it is run entirely under
> mmap_sem, but 99% of what it does doesn't actually need the lock.
> 
> The part that really needs the lock is
> 
>         addr = get_unmapped_area(file, addr, len, pgoff, flags);
>         addr = mmap_region(file, addr, len, vm_flags, pgoff);
> 
> but we hold it over all the other stuff too.
> 

True. By looking at the callers, we're always doing:

down_write(&mm->mmap_sem);
do_mmap_pgoff()
...
up_write(&mm->mmap_sem);

That goes for shm, aio, and of course mmap_pgoff().

While I know you hate two level locking, one way to go about this is to
take the lock inside do_mmap_pgoff() after the initial checks (flags,
page align, etc.) and return with the lock held, leaving the caller to
unlock it. 

> In fact, even if we moved the mmap_sem down into do_mmap(), and moved
> code around a bit to only hold it over those functions, it would still
> cover unnecessarily much. For example, while merging is common, not
> merging is pretty common too, and we do that
> 
>         vma = kmem_cache_zalloc(vm_area_cachep, GFP_KERNEL);
> 
> allocation under the lock. We could easily do things like preallocate
> it outside the lock.
> 

AFAICT there are also checks that should be done at the beginning of the
function, such as checking for MAP_LOCKED and VM_LOCKED flags before
calling get_unmapped_area().

Thanks,
Davidlohr

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