Re: [PATCH 0/3] mm,vdso: preallocate new vmas

[Davidlohr Bueso]

On Tue, 2013-10-22 at 08:48 -0700, walken@xxxxxxxxxx wrote:
> On Thu, Oct 17, 2013 at 05:50:35PM -0700, Davidlohr Bueso wrote:
> > Linus recently pointed out[1] some of the amount of unnecessary work 
> > being done with the mmap_sem held. This patchset is a very initial 
> > approach on reducing some of the contention on this lock, and moving
> > work outside of the critical region.
> > 
> > Patch 1 adds a simple helper function.
> > 
> > Patch 2 moves out some trivial setup logic in mlock related calls.
> > 
> > Patch 3 allows managing new vmas without requiring the mmap_sem for
> > vdsos. While it's true that there are many other scenarios where
> > this can be done, few are actually as straightforward as this in the
> > sense that we *always* end up allocating memory anyways, so there's really
> > no tradeoffs. For this reason I wanted to get this patch out in the open.
> > 
> > There are a few points to consider when preallocating vmas at the start
> > of system calls, such as how many new vmas (ie: callers of split_vma can
> > end up calling twice, depending on the mm state at that point) or the probability
> > that we end up merging the vma instead of having to create a new one, like the 
> > case of brk or copy_vma. In both cases the overhead of creating and freeing
> > memory at every syscall's invocation might outweigh what we gain in not holding
> > the sem.
> 
> Hi Davidlohr,
> 
> I had a quick look at the patches and I don't see anything wrong with them.
> However, I must also say that I have 99 problems with mmap_sem and the one
> you're solving doesn't seem to be one of them, so I would be interested to
> see performance numbers showing how much difference these changes make.
> 

Thanks for taking a look, could I get your ack? Unless folks have any
problems with the patchset, I do think it's worth picking up...

As discussed with Ingo, the numbers aren't too exciting, mainly because
of the nature of vdsos: with all patches combined I'm getting roughly
+5% throughput boost on some aim7 workloads. This was a first attempt at
dealing with the new vma allocation we do with the mmap_sem held. I've
recently explored some of the options I mentioned above, like for
brk(2). Unfortunately I haven't gotten good results; for instance we get
a -15% hit in ops/sec with aim9's brk_test program. So we end up using
already existing vmas a lot more than creating new ones, which makes a
lot of sense anyway. So all in all I don't think that preallocating vmas
outside the lock is a path to go, even at a micro-optimization level,
except for vdsos.

> Generally the problems I see with mmap_sem are related to long latency
> operations. Specifically, the mmap_sem write side is currently held
> during the entire munmap operation, which iterates over user pages to
> free them, and can take hundreds of milliseconds for large VMAs. Also,
> the mmap_sem read side is held during user page fauls - well, the
> VM_FAULT_RETRY mechanism allows us to drop mmap_sem during major page
> faults, but it is still held while allocating user pages or page tables,
> and while going through FS code for asynchronous readahead, which turns
> out not to be as asynchronous as you'd think as it can still block for
> reading extends etc... So, generally the main issues I am seeing with
> mmap_sem are latency related, while your changes only help for throughput
> for workloads that don't hit the above latency issues. I think that's a
> valid thing to do but I'm not sure if common workloads hit these throughput
> issues today ?

IMHO munmap is just one more example where having one big fat lock for
serializing an entire address space is something that must be addressed
(and, as you mention, mmap_sem doesn't even limit itself only to vmas -
heck... we even got it protecting superpage faults). For instance, it
would be really nice if we could somehow handle rbtrees with RCU, and
avoid taking any locks for things like lookups (ie: find_vma), of course
we need to guarantee that the vma won't be free'd underneath us one it's
found :) Or event allow concurrent updates and rebalancing - nonetheless
I don't know how realistic such things can be in the kernel.

> 
> > [1] https://lkml.org/lkml/2013/10/9/665 
> 
> Eh, that post really makes it look easy doesn't it :)
> 
> There are a few complications with mmap_sem as it turns out to protect
> more than just the VMA structures. For example, mmap_sem plays a role
> in preventing page tables from being unmapped while follow_page_mask()
> reads through them (there are other arch specific ways to do that,
> like disabling local interrupts on x86 to prevent TLB shootdown, but
> none that are currently available in generic code). This isn't an
> issue with your current proposed patches but is something you need to
> be aware of if you're going to do more work around the mmap_sem issues
> (which I would encourage you to BTW - there are a lot of issues around
> mmap_sem, so it definitely helps to have more people looking at this :)

Thanks for shedding some light on the topic!

Davidlohr


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