| Date | Tue, 12 Nov 2019 10:22:46 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch V2 08/16] x86/ioperm: Add bitmap sequence number |
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On Mon, Nov 11, 2019 at 11:03:22PM +0100, Thomas Gleixner wrote: > Add a globally unique sequence number which is incremented when ioperm() is > changing the I/O bitmap of a task. Store the new sequence number in the > io_bitmap structure and compare it along with the actual struct pointer > with the one which was last loaded on a CPU. Only update the bitmap if > either of the two changes. That should further reduce the overhead of I/O > bitmap scheduling when there are only a few I/O bitmap users on the system.
> + /* Update the sequence number to force an update in switch_to() */ > + iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence);
> + if (tss->last_bitmap != iobm || > + tss->last_sequence != iobm->sequence) > + switch_to_update_io_bitmap(tss, iobm);
Initially I wondered why we need a globally unique sequence number if we already check the struct iobitmap pointer. That ought to make the sequence number per-object.
However, that breaks for memory re-use. So yes, we need that thing to be global.
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