From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4A49D935.3060900@domain.hid> Date: Tue, 30 Jun 2009 11:21:57 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <4A48FB71.6070506@domain.hid> <4A49CD81.4060706@domain.hid> <4A49CFF0.7070202@domain.hid> <1246353623.7803.21.camel@domain.hid> In-Reply-To: <1246353623.7803.21.camel@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] x86: Endless minor faults List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum Cc: Jan Kiszka , xenomai-core Philippe Gerum wrote: > On Tue, 2009-06-30 at 10:42 +0200, Gilles Chanteperdrix wrote: >> Jan Kiszka wrote: >>> Jan Kiszka wrote: >>>> Hi all, >>>> >>>> seen such loops before? This particular trace is from a 2.6.29.3 kernel >>>> with ipipe-2.3-01 (SMP/PREEMPT_VOLUNTARY), but the same happens with >>>> 2.6.29.5/2.3-03: >>>> >>>> :| +func -653 0.084 __ipipe_handle_exception+0x11 (page_fault+0x26) >>>> :| +func -653 0.096 ipipe_check_context+0xd (__ipipe_handle_exception+0x71) >>>> :| #end 0x80000000 -653 0.069 do_page_fault+0x33 (__ipipe_handle_exception+0x1ff) >>>> : #func -653 0.078 __ipipe_unstall_root+0x9 (do_page_fault+0x3cb) >>>> :| #begin 0x80000000 -653 0.068 __ipipe_unstall_root+0x34 (do_page_fault+0x3cb) >>>> :| +end 0x80000000 -653 0.069 __ipipe_unstall_root+0x59 (do_page_fault+0x3cb) >>>> : +func -653 0.060 down_read_trylock+0x4 (do_page_fault+0x424) >>>> : +func -653 0.068 _spin_lock_irqsave+0x9 (__down_read_trylock+0x16) >>>> : +func -653 0.108 ipipe_check_context+0xd (_spin_lock_irqsave+0x1d) >>>> : #func -652 0.066 _spin_unlock_irqrestore+0x4 (__down_read_trylock+0x3f) >>>> : #func -652 0.069 __ipipe_restore_root+0x4 (_spin_unlock_irqrestore+0x21) >>>> : #func -652 0.074 __ipipe_unstall_root+0x9 (__ipipe_restore_root+0x2c) >>>> :| #begin 0x80000000 -652 0.066 __ipipe_unstall_root+0x34 (__ipipe_restore_root+0x2c) >>>> :| +end 0x80000000 -652 0.069 __ipipe_unstall_root+0x59 (__ipipe_restore_root+0x2c) >>>> : +func -652 0.096 find_vma+0x4 (do_page_fault+0x465) >>>> : +func -652 0.150 ltt_run_filter_default+0x4 (_ltt_specialized_trace+0xc1) >>>> : +func -652 0.098 handle_mm_fault+0x11 (do_page_fault+0x537) >>>> : +func -652 0.090 _spin_lock+0x4 (handle_mm_fault+0x680) >>>> : +func -652 0.063 ptep_set_access_flags+0x9 (handle_mm_fault+0x6d1) >>>> : +func -652 0.282 flush_tlb_page+0xd (handle_mm_fault+0x6e7) >>>> : +func -651 0.162 ltt_run_filter_default+0x4 (_ltt_specialized_trace+0xc1) >>>> : +func -651 0.062 up_read+0x4 (do_page_fault+0x5a9) >>>> : +func -651 0.072 _spin_lock_irqsave+0x9 (__up_read+0x1c) >>>> : +func -651 0.117 ipipe_check_context+0xd (_spin_lock_irqsave+0x1d) >>>> : #func -651 0.074 _spin_unlock_irqrestore+0x4 (__up_read+0x92) >>>> : #func -651 0.069 __ipipe_restore_root+0x4 (_spin_unlock_irqrestore+0x21) >>>> : #func -651 0.060 __ipipe_unstall_root+0x9 (__ipipe_restore_root+0x2c) >>>> :| #begin 0x80000000 -651 0.056 __ipipe_unstall_root+0x34 (__ipipe_restore_root+0x2c) >>>> :| +end 0x80000000 -651 0.420 __ipipe_unstall_root+0x59 (__ipipe_restore_root+0x2c) >>>> :| +func -650 0.084 __ipipe_handle_exception+0x11 (page_fault+0x26) >>>> >>>> and again and again... >>>> >>>> We are looping over a minor fault here (according to /proc/PID/stat), >>>> the context is a Xenomai task in secondary mode. As the task no longer >>>> processes signals in this state, the whole system is more or less >>>> broken. Tomorrow I will try to find out the faulting address with an >>>> instrumented kernel, but maybe you already have some ideas. >>> The fault is apparently triggered by __xn_put_user(XNRELAX, >>> thread->u_mode) in xnshadow_relax. thread->u_mode is pointing to an >>> invalid region ATM. The questions are now: Who corrupted this, user >>> space on init (not that likely) or kernel space later on (unpleasant >>> thought)? Moreover: Why can't we recover from a fault on u_mode? >> I already investigated such an issue, and my conclusion was that there >> are some places in the code where we can not cope with a fault. >> xnshadow_relax being such a place, because, if relax faults, then what >> will the fault handler do? Call relax again. Fortunately, mlockall and >> the nocow stuff fixes this. > > > xnshadow_relax() faulting before the current thread bears the XNRELAX > bit would mean that a creepy issue involving ondemand PTEs in _kernel_ > space must have caused this. Having the init_mm mappings known from all > processes seems more relevant to this issue than anything nocow and/or > mlockall could ever do to fix it. u_mode is a user-space address. -- Gilles