Re: [discuss] f_ops flag to speed up compatible ioctls in linux kernel

[Andi Kleen]

On Tue, Sep 07, 2004 at 04:45:18PM +0300, Michael S. Tsirkin wrote:
> > > I built a silly driver example which just used a semaphore and a switch
> > > statement inside the ioctl.
> > > 
> > > ~/<1>tavor/tools/driver_new>time /tmp/ioctltest64 /dev/mst/mt23108_pci_cr0
> > > 0.357u 4.760s 0:05.11 100.0%    0+0k 0+0io 0pf+0w
> > > ~/<1>tavor/tools/driver_new>time /tmp/ioctltest32 /dev/mst/mt23108_pci_cr0
> > > 0.641u 6.486s 0:07.12 100.0%    0+0k 0+0io 0pf+0w
> > > 
> > > So just looking at system time there seems to be an overhead of
> > > about 20%.
> > 
> > That's with an empty ioctl?
> 
> Not exactly empty - below's the code snippet.

Hmm, ok. Surprising then. Can you profile it to see where 
the bottleneck exactly is? 

> > I would expect most ioctls to do
> > more work, so the overhead would be less.
> > Still it could be probably made better. 
> 
> Then I expect you'll get bitten by the BKL taken while ioctl runs.

Yes, but that's a general problem, not specific to compat ioctls.

So far nobody dared to drop the BKL for ioctls because it would
require to audit/fix a *lot* of code.

The idea of taking the BKL during the hash lookup was that
when the BKL is taken anyways it doesn't make too much 
difference to take it a little bit longer. But this assumed
that the hash lookup is fast. If it isn't maybe the hash
function should just be optimized a bit or the table increased.

Most of the values are known at compile time, so maybe
some perfect hash generator like gperf could be used to
generate a better hash? 


> >
> > In theory the BKL could be dropped from the lookup anyways
> > if RCU is needed for the cleanup. For locking the handler 
> > itself into memory it doesn't make any difference.
> > 
> > What happens when you just remove the lock_kernel() there? 
> > (as long as you don't unload any modules this should be safe) 
> > 
> > -Andi
> 
> Well, I personally do want to enable module unloading.

It works fine as long as the compat function is in the same
module as the one providing the file_ops.

> I think I'll add a new entry point to f_ops  and see what *this* does
> to speed. That would be roughly equivalent, and cleaner, right?

It may help your module, but won't solve the general problem shorter
term.

-Andi

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