Date: Wed, 22 Jan 2014 15:27:12 -0500 From: John Baldwin <jhb@freebsd.org> To: Alfred Perlstein <alfred@freebsd.org> Cc: src-committers@freebsd.org, Scott Long <scott4long@yahoo.com>, Neel Natu <neel@freebsd.org>, John-Mark Gurney <jmg@funkthat.com>, svn-src-all@freebsd.org, Rui Paulo <rpaulo@felyko.com>, svn-src-head@freebsd.org, Alexander Kabaev <kabaev@gmail.com> Subject: Re: svn commit: r260898 - head/sys/kern Message-ID: <201401221527.12779.jhb@freebsd.org> In-Reply-To: <52E016BF.80102@freebsd.org> References: <201401200159.s0K1xa5X012123@svn.freebsd.org> <20140122181443.GU75135@funkthat.com> <52E016BF.80102@freebsd.org>
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On Wednesday, January 22, 2014 2:06:39 pm Alfred Perlstein wrote: > > On 1/22/14, 10:14 AM, John-Mark Gurney wrote: > > Scott Long wrote this message on Tue, Jan 21, 2014 at 15:12 -0700: > >> On Jan 21, 2014, at 9:26 AM, John Baldwin <jhb@freebsd.org> wrote: > >> > >>> On Monday, January 20, 2014 5:18:44 pm Alexander Kabaev wrote: > >>>> On Mon, 20 Jan 2014 11:32:29 -0500 > >>>> John Baldwin <jhb@freebsd.org> wrote: > >>>> > >>>>> On Sunday 19 January 2014 18:18:03 Rui Paulo wrote: > >>>>>> On 19 Jan 2014, at 17:59, Neel Natu <neel@FreeBSD.org> wrote: > >>>>>>> Author: neel > >>>>>>> Date: Mon Jan 20 01:59:35 2014 > >>>>>>> New Revision: 260898 > >>>>>>> URL: http://svnweb.freebsd.org/changeset/base/260898 > >>>>>>> > >>>>>>> Log: > >>>>>>> Bump up WITNESS_COUNT from 1024 to 1536 so there are sufficient > >>>>>>> entries for > >>>>>>> WITNESS to actually work. > >>>>>> This value should be automatically tuned... > >>>>> How do you propose to do so? This is the count of locks initialized > >>>>> before witness' own SYSINIT is executed and the array it sizes is > >>>>> allocated statically at compile time. This used to not be a static > >>>>> array, but an intrusive list embedded in locks themselves, but we > >>>>> decided to shave a pointer off of each lock that was only used for > >>>>> that and to use a statically sized table instead. > >>>>> > >>>>> -- > >>>>> John Baldwin > >>>> As <CONSTANT1> + <CONSTANT2> * MAXCPU, as evidently most recent > >>>> overflows reported were caused by jacking MAXCPU up from its default > >>>> value? > >>> If raising MAXCPU changes the number of unique lock names used, then the > >>> locks are named incorrectly. We don't use the 'pid' in the name for > >>> PROC_LOCK precisely so that WITNESS will treat them all the same so > >>> that if if it learns a lock order for pid 37 it enforces the same lock > >>> order for pid 38. Device locks should follow a similar rule. They > >>> should generally not include the device name (and in some cases they > >>> really shouldn't even have the driver name). > >> Why shouldn?t they have a driver and device name? Wouldn?t it help identify > >> possible deadlocks from driver instances calling into each other? > > Locks have a name and a type. The type is used for witness, but if it > > is NULL, the name is used. So you could if you wanted, create a common > > type, and then put driver/device name in name, but the passed in strings > > to both name and type have to be stable storage (only the pointer is > > stored), so you can't use a stack variable to construct it. > > > Hmm, what if locks had a pointer to a 2 element char * array, the first > being the name, the second the type. That would keep the size of the > lock down and most locks could share a common tuple of name/type in each > subsystem. This would allow us to get rid of the pending static list. > > effectively: > struct lock_object { > char *lo_name; /* Individual lock name. */ > u_int lo_flags; > u_int lo_data; /* General class specific data. */ > struct witness *lo_witness; /* Data for witness. */ > }; > > would change to: > struct lock_object { > char **lo_name_type; /* Individual lock > name[0]/type[1]. */ > u_int lo_flags; > u_int lo_data; /* General class specific data. */ > struct witness *lo_witness; /* Data for witness. */ > }; > > This may be somewhat disruptive, I haven't played with how it would > actually change driver/etc/code. Where would the memory for the char* array come from? -- John Baldwin
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