Re: [RFC PATCH] x86-64: software IRQ masking and handling

[Ingo Molnar <mingo@elte.hu>]

* Tejun Heo <tj@kernel.org> wrote:

> Hello,
> 
> This is something suggested by Rusty Russell a while ago.  It makes IRQ 
> masking a software switch like preemption or softirq enable/disable.  
> Hardware interrupt masking (cli/sti) and delivery are decoupled from actual 
> IRQ handling.  IRQ disabling is done by single instruction moving 1 to a 
> percpu variable.  Enabling is similar but it should check whether there's 
> any pending interrupt to handle.
> 
> This change greatly reduces the number of hardware IRQ masking 
> manipulations.  cli/sti still being somewhat costly operations (I hear 
> nehalem is better tho), this should be able to improve overall performance, 
> especially on paravirts.

Not just Nehalem but on various AMD CPUs it was in the below-10-cycles range 
for years.

Note that we tried this in -rt, but the pain and trouble (and, often, code 
bloat) was not worth the trouble. The PUSHF/POPF/CLI/STI instructions are 
really simple and short in the instruction stream - without disturbing other 
registers.

	Ingo