On Thu, 1 Nov 2012, Dave Jones wrote: > On Thu, Nov 01, 2012 at 04:48:41PM -0700, Hugh Dickins wrote: > > > > Fedora turns on CONFIG_DEBUG_VM? > > Yes. > > > All mm developers should thank you for the wider testing exposure; > > but I'm not so sure that Fedora users should thank you for turning > > it on - really it's for mm developers to wrap around !assertions or > > more expensive checks (e.g. checking calls) in their development. > > The last time I did some benchmarking the impact wasn't as ridiculous > as say lockdep, or spinlock debug. I think you're safe to assume that (outside of an individual developer's private tree) it will never be nearly as heavy as lockdep or debug pagealloc. I hadn't thought of spinlock debug as a heavy one, but yes, I guess it would be heavier than almost all VM_BUG_ON()s. > Maybe the benchmarks I was using > weren't pushing the VM very hard, but it seemed to me that the value > in getting info in potential problems early was higher than a small > performance increase. We thank you. I may have been over-estimating how much we put inside those VM_BUG_ON()s, sorry. Just so long as you're aware that there's a danger that one day we might slip something heavier in there. Those few explicit #ifdef CONFIG_DEBUG_VMs sometimes found in mm/ are probably the worst: you might want to check on the current crop. > > > Or did I read a few months ago that some change had been made to > > such definitions, and VM_BUG_ON(contents) are evaluated even when > > the config option is off? I do hope I'm mistaken on that. > > Pretty sure that isn't the case. I remember Andrew chastising people > a few times for putting checks in VM_BUG_ON's that needed to stay around > even when the config option was off. Perhaps you were thinking of one > of those incidents ? Avoiding side-effects in BUG_ON and VM_BUG_ON. Yes, that comes up from time to time, and I'm a believer on that. I think the discussion I'm mis/remembering sprung out of one of those: someone was surprised by the disassembly they found when it was configured off. The correct answer is to try it for myself and see. Not today. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>