Messages in this thread | | | Date | Tue, 7 Sep 2004 16:15:24 +0200 | From | Andi Kleen <> | Subject | Re: [discuss] f_ops flag to speed up compatible ioctls in linux kernel |
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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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