Messages in this thread | | | Date | Thu, 05 Dec 2002 14:15:39 -0800 | From | Andrew Morton <> | Subject | Re: [patch] kmalloc_percpu -- 2 of 2 |
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Dipankar Sarma wrote: > > On Thu, Dec 05, 2002 at 09:10:16PM +0100, Andrew Morton wrote: > > > > I'd suggest that you drop the new allocator until a compelling > > need for it (in real, live 2.5/2.6 code) has been demonstrated. > > Fine with me since atleast one workaround for fragmentation with small > allocations is known. I can't see anything in 2.5 timeframe > requiring small per-cpu allocations. > > Would you like me to resubmit a simple kmalloc-only version ? >
I think that would be best.
BTW, looking at the snmp application of this work:
+#define ICMP_INC_STATS_USER_FIELD(offt) \ + (*((unsigned long *) ((void *) \ + per_cpu_ptr(icmp_statistics[1], \ + smp_processor_id())) + offt))++;
This guy is racy on preempt. Just a little bit. It is effectively:
ptr = per_cpu_ptr(...); (*ptr)++;
On some architectures, `(*ptr)++' is not atomic wrt interrupts. The CPU could be preempted midway through the increment.
Surely it's not an issue for SNMP stats, but for some applications such as struct page_state, such a permanent off-by-a-little-bit would be a showstopper.
So some big loud comments which describe the worthiness of get_cpu_ptr(), and the potential inaccuracy of per_cpu_ptr would be useful.
And as this is the first application of the kmalloc_precpu infrastructure, it may be best to convert it to use get_cpu_ptr/put_cpu_ptr. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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