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Date: Tue, 10 Nov 2009 23:09:19 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Hugh Dickins <hugh.dickins@...cali.co.uk> Cc: Andrew Morton <akpm@...ux-foundation.org>, Izik Eidus <ieidus@...hat.com>, Andrea Arcangeli <aarcange@...hat.com>, Christoph Lameter <cl@...ux-foundation.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH 5/6] mm: stop ptlock enlarging struct page On Tue, 2009-11-10 at 22:02 +0000, Hugh Dickins wrote: > CONFIG_DEBUG_SPINLOCK adds 12 or 16 bytes to a 32- or 64-bit spinlock_t, > and CONFIG_DEBUG_LOCK_ALLOC adds another 12 or 24 bytes to it: lockdep > enables both of those, and CONFIG_LOCK_STAT adds 8 or 16 bytes to that. > > When 2.6.15 placed the split page table lock inside struct page (usually > sized 32 or 56 bytes), only CONFIG_DEBUG_SPINLOCK was a possibility, and > we ignored the enlargement (but fitted in CONFIG_GENERIC_LOCKBREAK's 4 > by letting the spinlock_t occupy both page->private and page->mapping). > > Should these debugging options be allowed to double the size of a struct > page, when only one minority use of the page (as a page table) needs to > fit a spinlock in there? Perhaps not. > > Take the easy way out: switch off SPLIT_PTLOCK_CPUS when DEBUG_SPINLOCK > or DEBUG_LOCK_ALLOC is in force. I've sometimes tried to be cleverer, > kmallocing a cacheline for the spinlock when it doesn't fit, but given > up each time. Falling back to mm->page_table_lock (as we do when ptlock > is not split) lets lockdep check out the strictest path anyway. Why? we know lockdep bloats stuff we never cared.. and hiding a popular CONFIG option from lockdep doesn't seem like a good idea to me. > Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk> > --- > > mm/Kconfig | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > --- mm4/mm/Kconfig 2009-11-04 10:52:58.000000000 +0000 > +++ mm5/mm/Kconfig 2009-11-04 10:53:13.000000000 +0000 > @@ -161,11 +161,13 @@ config PAGEFLAGS_EXTENDED > # Default to 4 for wider testing, though 8 might be more appropriate. > # ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock. > # PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes. > +# DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC spinlock_t also enlarge struct page. > # > config SPLIT_PTLOCK_CPUS > int > - default "4096" if ARM && !CPU_CACHE_VIPT > - default "4096" if PARISC && !PA20 > + default "999999" if ARM && !CPU_CACHE_VIPT > + default "999999" if PARISC && !PA20 > + default "999999" if DEBUG_SPINLOCK || DEBUG_LOCK_ALLOC > default "4" > > # -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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