On Fri, Apr 5, 2024 at 5:44 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Thu, Apr 04, 2024 at 07:00:51PM -0400, Kent Overstreet wrote: > > On Thu, Apr 04, 2024 at 03:41:50PM -0700, Andrew Morton wrote: > > > On Thu, 4 Apr 2024 18:38:39 -0400 Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote: > > > > > > > On Thu, Apr 04, 2024 at 11:33:22PM +0100, Matthew Wilcox wrote: > > > > > On Thu, Apr 04, 2024 at 03:17:43PM -0700, Suren Baghdasaryan wrote: > > > > > > Ironically, checkpatch generates warnings for these type casts: > > > > > > > > > > > > WARNING: unnecessary cast may hide bugs, see > > > > > > http://c-faq.com/malloc/mallocnocast.html > > > > > > #425: FILE: include/linux/dma-fence-chain.h:90: > > > > > > + ((struct dma_fence_chain *)kmalloc(sizeof(struct dma_fence_chain), > > > > > > GFP_KERNEL)) > > > > > > > > > > > > I guess I can safely ignore them in this case (since we cast to the > > > > > > expected type)? > > > > > > > > > > I find ignoring checkpatch to be a solid move 99% of the time. > > > > > > > > > > I really don't like the codetags. This is so much churn, and it could > > > > > all be avoided by just passing in _RET_IP_ or _THIS_IP_ depending on > > > > > whether we wanted to profile this function or its caller. vmalloc > > > > > has done it this way since 2008 (OK, using __builtin_return_address()) > > > > > and lockdep has used _THIS_IP_ / _RET_IP_ since 2006. > > > > > > > > Except you can't. We've been over this; using that approach for tracing > > > > is one thing, using it for actual accounting isn't workable. > > > > > > I missed that. There have been many emails. Please remind us of the > > > reasoning here. > > > > I think it's on the other people claiming 'oh this would be so easy if > > you just do it this other way' to put up some code - or at least more > > than hot takes. > > Well, /proc/vmallocinfo exists, and has existed since 2008, so this is > slightly more than a "hot take". > > > But, since you asked - one of the main goals of this patchset was to be > > fast enough to run in production, and if you do it by return address > > then you've added at minimum a hash table lookup to every allocate and > > free; if you do that, running it in production is completely out of the > > question. > > And yet vmalloc doesn't do that. > > > Besides that - the issues with annotating and tracking the correct > > callsite really don't go away, they just shift around a bit. It's true > > that the return address approach would be easier initially, but that's > > not all we're concerned with; we're concerned with making sure > > allocations get accounted to the _correct_ callsite so that we're giving > > numbers that you can trust, and by making things less explicit you make > > that harder. > > I'm not convinced that _THIS_IP_ is less precise than a codetag. They > do essentially the same thing, except that codetags embed the source > location in the file while _THIS_IP_ requires a tool like faddr2line > to decode kernel_clone+0xc0/0x430 into a file + line number. > > > This is all stuff that I've explained before; let's please dial back on > > the whining - or I'll just bookmark this for next time... > > Please stop mischaracterising serious thoughtful criticism as whining. > I don't understand what value codetags bring over using _THIS_IP_ and > _RET_IP_ and you need to explain that. The conceptual difference between codetag and _THIS_IP_/_RET_IP_ is that codetag injects counters at the call site, so you don't need to spend time finding the appropriate counter to operate on during allocation.
