Mathieu Desnoyers <[email protected]> writes:
> Thread Migration Preemption
>
> This patch adds the ability to protect critical sections from migration to
> another CPU without disabling preemption.
Good idea.
I sometimes think we could have avoided _much_ trouble
if that had been always default for processes running
in kernel space.
> This will be useful to minimize the amount of preemption disabling for the -rt
> patch. It will help leveraging improvements brought by the local_t types in
> asm/local.h (see Documentation/local_ops.txt). Note that the updates done to
> variables protected by migration_disable must be either atomic or protected from
> concurrent updates done by other threads.
>
> Typical use:
>
> migration_disable();
> local_inc(&__get_cpu_var(&my_local_t_var));
> migration_enable();
It seems strange to have a new interface for this. We already
have get_cpu()/put_cpu(). So why not use that?
> unsigned long flags; /* low level flags */
> __u32 cpu;
> __s32 preempt_count; /* 0 => preemptable, <0 => BUG */
> + int migration_count;/* 0: can migrate, <0 => BUG */
Can you turn preempt_count into a short first and use a short? That should be enough
and cache line usage wouldn't be increased. That's ok on x86; on RISCs
int might be faster
-Andi
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