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SubjectRe: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical

* Thomas Gleixner <tglx@linutronix.de> wrote:

> The I/O bitmap is duplicated on fork. That's wasting memory and slows down
> fork. There is no point to do so. As long as the bitmap is not modified it
> can be shared between threads and processes.
>
> Add a refcount and just share it on fork. If a task modifies the bitmap
> then it has to do the duplication if and only if it is shared.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> V2: New patch
> ---
> arch/x86/include/asm/iobitmap.h | 5 +++++
> arch/x86/kernel/ioport.c | 38 ++++++++++++++++++++++++++++++++------
> arch/x86/kernel/process.c | 39 ++++++---------------------------------
> 3 files changed, 43 insertions(+), 39 deletions(-)
>
> --- a/arch/x86/include/asm/iobitmap.h
> +++ b/arch/x86/include/asm/iobitmap.h
> @@ -2,10 +2,12 @@
> #ifndef _ASM_X86_IOBITMAP_H
> #define _ASM_X86_IOBITMAP_H
>
> +#include <linux/refcount.h>
> #include <asm/processor.h>
>
> struct io_bitmap {
> u64 sequence;
> + refcount_t refcnt;
> unsigned int io_bitmap_max;
> union {
> unsigned long bits[IO_BITMAP_LONGS];

> +void io_bitmap_share(struct task_struct *tsk)
> + {
> + /*
> + * Take a refcount on current's bitmap. It can be used by
> + * both tasks as long as none of them changes the bitmap.
> + */
> + refcount_inc(&current->thread.io_bitmap->refcnt);
> + tsk->thread.io_bitmap = current->thread.io_bitmap;
> + set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
> +}

Ok, this is really neat. I suspect there might be some pathological cases
on ancient NUMA systems with a really high NUMA factor and bad caching
where this new sharing might regress performance, but I doubt this
matters, as both the hardware and this software functionality is legacy.


> + /*
> + * If the bitmap is not shared, then nothing can take a refcount as
> + * current can obviously not fork at the same time. If it's shared
> + * duplicate it and drop the refcount on the original one.
> + */
> + if (refcount_read(&iobm->refcnt) > 1) {
> + iobm = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL);
> + if (!iobm)
> + return -ENOMEM;
> + io_bitmap_exit();
> }
>
> + /* Set the tasks io_bitmap pointer (might be the same) */

speling nit:

s/tasks
/task's

Thanks,

Ingo

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