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SubjectRe: SMP spin-locks
I thought that when you compiled a kernel as UP it replaced the spin-lock
macros with versions that are blank. As a result a UP kernel spends no
time doing spinlocks at all.

that's why a SMP kernel on a UP box is slightly slower, there is more code
to be executed

David Lang



On Thu, 14 Jun 2001, Richard B. Johnson wrote:

> Date: Thu, 14 Jun 2001 23:21:35 -0400 (EDT)
> From: Richard B. Johnson <root@chaos.analogic.com>
> To: Roger Larsson <roger.larsson@norran.net>
> Cc: Linux kernel <linux-kernel@vger.kernel.org>
> Subject: Re: SMP spin-locks
>
> On Thu, 14 Jun 2001, Roger Larsson wrote:
>
> > On Thursday 14 June 2001 23:05, you wrote:
> > > On Thu, 14 Jun 2001, Roger Larsson wrote:
> > > > Hi,
> > > >
> > > > Wait a minute...
> > > >
> > > > Spinlocks on a embedded system? Is it _really_ SMP?
> > >
> > > The embedded system is not SMP. However, there is definite
> > > advantage to using an unmodified kernel that may/may-not
> > > have been compiled for SMP. Of course spin-locks are used
> > > to prevent interrupts from screwing up buffer pointers, etc.
> > >
> >
> > Not really - it prevents another processor entering the same code
> > segment (spin_lock_irqsave prevents both another processor and
> > local interrupts).
> >
> > An interrupt on UP can not wait on a spin lock - it will never be released
> > since no other code than the interrupt spinning will be able to execute)
>
> An interrupt on a UP system will never spin, nor will the IP from
> another CPU because there isn't another CPU. A spin-lock, compiled
> for UP is:
>
> pushf
> popl some_register, currently EBX
> cli ; Clear the interrupts on the only CPU you have
>
> do_some_code_that_must_not_be_interrupted();
>
> pushl same_register_as_above
> popf ; Restore interrupts if they were enabled
>
>
> For SMP is:
>
> pushf
> popl some_register
> cli ; Clear interrupts
> modify_a_memory_variable
> x: see_if_it_is_what_you_expect
> if_not_loop_to x
>
> do_some_code_that_must_not_be_interrupted();
>
> modify_the_memory_variable_back
> pushl same_register_as_above
> popf
>
>
> Since `cli` will only stop interrupts on the CPU that actually
> fetches the instruction, another CPU can enter the code unless
> it is forced to spin until the lock is released.
>
> If this code is executed on a UP machine, the memory variable
> will always become exactly as expected so it will never spin.
> Therefore SMP code should be perfectly safe on a UP machine,
> in fact must be perfectly safe, or it's broken.
>
> The current spinlock code does work perfectly on a UP machine.
> However, the large difference in performance shows that something
> is quite less than optimum in the coding.
>
> Spinlocks are machine dependent. A simple increment of a byte
> memory variable, spinning if it's not 1 will do fine. Decrementing
> this variable will release the lock. A `lock` prefix is not necessary
> because all Intel byte operations are atomic anyway. This assumes
> that the lock was initialized to 0. It doesn't have to be. It
> could be initialized to 0xaa (anything) and spin if it's not
> 0xab (or anything + 1).
>
>
> >
> > SMP compiled kernel, but running on UP hardware - right?
> > Then this _should not_ happen!
> >
> > see linux/Documentation/spinlocks.txt
> >
>
> This, in fact, will happen. Machines booted from the network should
> have SMP code so a SMP machine can use all its CPUs. This same
> code, booted from the network, should have no measurable performance
> penalty in UP machines.
>
> Also, when you develop drivers on a workstation, test them on
> a workstation, then upload everything to an embedded system, you
> had better be executing the same code, kernel, drivers, et all,
> or you are in a world of hurt. Many embedded systems don't have
> any 'standard I/O' so you can't prove that it meets its specs
> (exception handling, etc) on the target. You have to test that
> logic elsewhere.
>
> This workstation has two CPUs. All drivers are modules. It uses
> initrd to install the ones for my SCSI disks, network, etc.
>
> Script started on Thu Jun 14 23:13:10 2001
> lsmod
> Module Size Used by
> ramdisk 4448 0
> loop 8212 0 (autoclean)
> ipx 19248 0 (unused)
> 3c59x 25020 1 (autoclean)
> nls_cp437 4408 4 (autoclean)
> BusLogic 38320 6
> sd_mod 10932 6
> scsi_mod 59460 2 [BusLogic sd_mod]
> # exit
> exit
>
> Script done on Thu Jun 14 23:13:45 2001
>
> The same kernel, uploaded to an embedded system, also uses
> initrd to load the machine-specific drivers. In this way, only
> the drivers that are actually used, are loaded. The kernel remains
> small. There is a slight performance penality for using modules,
> but none other.
>
> # telnet platinum
> Trying 10.106.100.166...
> Connected to platinum.analogic.com.
> Escape character is '^]'.
>
> Enter "help" for commands
>
> PLATINUM> sho modules
>
> pcilynx 13468 1
> raw1394 7984 1
> ieee1394 22984 0 [pcilynx raw1394]
> rtc_drvr 2372 0
> vxibus 10660 6
> gpib_drvr 19200 2
> ramdisk 4428 0
> pcnet32se 15640 1
>
> PLATINUM> exit
> Exit
>
> Connection closed by foreign host.
> # exit
> exit
>
>
> Cheers,
> Dick Johnson
>
> Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).
>
> "Memory is like gasoline. You use it up when you are running. Of
> course you get it all back when you reboot..."; Actual explanation
> obtained from the Micro$oft help desk.
>
>
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