From: "Michael S. Tsirkin" <mst@mellanox.co.il>
To: Andi Kleen <ak@suse.de>
Cc: discuss@x86-64.org, linux-kernel@vger.kernel.org
Subject: Re: [discuss] f_ops flag to speed up compatible ioctls in linux kernel
Date: Tue, 7 Sep 2004 13:40:17 +0300 [thread overview]
Message-ID: <20040907104017.GB10096@mellanox.co.il> (raw)
In-Reply-To: <20040903080058.GB2402@wotan.suse.de>
Hello!
Quoting Andi Kleen (ak@suse.de) "Re: [discuss] f_ops flag to speed up compatible ioctls in linux kernel":
> On Wed, Sep 01, 2004 at 10:22:45AM +0300, Michael S. Tsirkin wrote:
> > Hello!
> > Currently, on the x86_64 architecture, its quite tricky to make
> > a char device ioctl work for an x86 executables.
> > In particular,
> > 1. there is a requirement that ioctl number is unique -
> > which is hard to guarantee especially for out of kernel modules
>
> Yes, that is a problem for some people. But you should
> have used an unique number in the first place.
Do you mean the _IOC macro and friends?
But their uniqueness depends on allocating a unique magic number
in the first place.
> There are some hackish ways to work around it for non modules[1], but at some
> point we should probably support it better.
>
> [1] it can be handled, except for module unloading, so you have
> to disable that.
Why use the global hash at all?
Why not, for example, pass a parameter to the ioctl function
to make it possible to figure out this is a compat call?
> > 2. there's a performance huge overhead for each compat call - there's
> > a hash lookup in a global hash inside a lock_kernel -
> > and I think compat performance *is* important.
>
> Did you actually measure it? I doubt it is a big issue.
>
But that would depend on what the driver actually does inside
the ioctl and on how many ioctls are already registered, would it not?
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%.
The overhead is bigger if there are collisions in the hash.
For muti-processor scenarious, the difference is much more pronounced
(note I have dual-cpu Opteron system):
~>time /tmp/ioctltest32 /dev/mst/mt23108_pci_cr0 & ;time /tmp/ioctltest32
/dev/mst/mt23108_pci_cr0 &
[2] 10829
[3] 10830
[2] Done /tmp/ioctltest32 /dev/mst/mt23108_pci_cr0
0.435u 21.322s 0:21.76 99.9% 0+0k 0+0io 0pf+0w
[3] Done /tmp/ioctltest32 /dev/mst/mt23108_pci_cr0
0.683u 21.231s 0:21.92 99.9% 0+0k 0+0io 0pf+0w
~>
~>time /tmp/ioctltest64 /dev/mst/mt23108_pci_cr0 & ;time /tmp/ioctltest64
/dev/mst/mt23108_pci_cr0 &
[2] 10831
[3] 10832
[3] Done /tmp/ioctltest64 /dev/mst/mt23108_pci_cr0
0.474u 11.194s 0:11.70 99.6% 0+0k 0+0io 0pf+0w
[2] Done /tmp/ioctltest64 /dev/mst/mt23108_pci_cr0
0.476u 11.277s 0:11.75 99.9% 0+0k 0+0io 0pf+0w
~>
So we get 50% slowdown.
I imagine this is the result of BKL contention during the hash lookup.
MST
next prev parent reply other threads:[~2004-09-07 10:34 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-09-01 7:22 f_ops flag to speed up compatible ioctls in linux kernel Michael S. Tsirkin
2004-09-01 7:32 ` viro
2004-09-01 7:44 ` Michael S. Tsirkin
2004-09-01 7:47 ` Lee Revell
2004-09-01 8:19 ` Michael S. Tsirkin
2004-09-01 15:55 ` Roland Dreier
2004-09-01 18:02 ` Chris Wright
2004-09-01 18:12 ` Roland Dreier
2004-09-01 18:31 ` viro
2004-09-01 20:54 ` Roland Dreier
[not found] ` <20040901170800.K1924@build.pdx.osdl.net>
[not found] ` <20040901190122.L1924@build.pdx.osdl.net>
2004-09-02 3:46 ` Roland Dreier
2004-09-01 18:06 ` Bill Davidsen
2004-09-01 8:30 ` Arjan van de Ven
2004-09-01 15:40 ` [PATCH] fs/compat.c: rwsem instead of BKL around ioctl32_hash_table Roland Dreier
2004-09-01 23:27 ` Andrew Morton
2004-09-02 21:14 ` Andi Kleen
2004-09-02 22:26 ` Roland Dreier
2004-09-03 14:37 ` [discuss] " Andi Kleen
2004-09-03 14:55 ` Roland Dreier
2004-09-03 15:02 ` Andi Kleen
2004-09-03 8:00 ` [discuss] f_ops flag to speed up compatible ioctls in linux kernel Andi Kleen
2004-09-07 10:40 ` Michael S. Tsirkin [this message]
2004-09-07 12:14 ` Andi Kleen
2004-09-07 13:45 ` Michael S. Tsirkin
2004-09-07 14:15 ` Andi Kleen
2004-09-07 14:25 ` Michael S. Tsirkin
2004-09-07 14:29 ` Andi Kleen
2004-09-07 14:37 ` Michael S. Tsirkin
2004-09-07 14:44 ` Andi Kleen
2004-09-07 14:45 ` Michael S. Tsirkin
2004-09-07 15:10 ` Andi Kleen
2004-09-07 18:16 ` Michael S. Tsirkin
2004-09-08 6:55 ` Andi Kleen
2004-09-08 14:28 ` [patch] " Michael S. Tsirkin
2004-09-08 14:38 ` Andi Kleen
2004-09-08 14:54 ` Michael S. Tsirkin
2004-09-08 14:58 ` Andi Kleen
2004-09-12 20:05 ` Michael S. Tsirkin
2004-09-15 13:19 ` [patch] Re: [discuss] " Michael S. Tsirkin
2004-09-07 21:36 ` Is FIOQSIZE compatible? ( was Re: f_ops flag to speed up compatible ioctls in linux kernel) Michael S. Tsirkin
2004-09-08 6:54 ` Andi Kleen
2004-09-07 15:03 ` [discuss] f_ops flag to speed up compatible ioctls in linux kernel Herbert Poetzl
2004-09-07 18:07 ` Michael S. Tsirkin
2004-09-09 13:54 ` Herbert Poetzl
2004-12-12 21:51 ` how to detect a 32 bit process on 64 bit kernel Michael S. Tsirkin
2004-12-12 22:01 ` Jan Engelhardt
2004-12-12 22:23 ` Christoph Hellwig
2004-12-13 19:50 ` Michael S. Tsirkin
2004-12-13 21:01 ` Jan Engelhardt
2004-12-13 21:32 ` Brian Gerst
2004-12-13 21:37 ` Michael S. Tsirkin
2004-12-12 22:37 ` Willy Tarreau
2004-12-12 23:30 ` Bongani Hlope
2004-12-14 7:28 ` Andi Kleen
2004-09-20 14:49 ` [patch] speed up ioctls in linux kernel Michael S. Tsirkin
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