Re: [PATCH v2] pseries/drmem: don't cache node id in drmem_lmb struct

From: David Hildenbrand <david@redhat.com>
To: Scott Cheloha <cheloha@linux.ibm.com>, linuxppc-dev@ozlabs.org
Cc: Nathan Lynch <nathanl@linux.ibm.com>,
	Michal Suchanek <msuchanek@suse.de>,
	Laurent Dufour <ldufour@linux.vnet.ibm.com>,
	Rick Lindsley <ricklind@linux.vnet.ibm.com>
Subject: Re: [PATCH v2] pseries/drmem: don't cache node id in drmem_lmb struct
Date: Tue, 28 Jul 2020 20:22:11 +0200	[thread overview]
Message-ID: <2249e1b4-57f0-dfd3-1104-3e7cb2f90840@redhat.com> (raw)
In-Reply-To: <20200728165339.3031126-1-cheloha@linux.ibm.com>

On 28.07.20 18:53, Scott Cheloha wrote:
> At memory hot-remove time we can retrieve an LMB's nid from its
> corresponding memory_block.  There is no need to store the nid
> in multiple locations.
> 
> Note that lmb_to_memblock() uses find_memory_block() to get the
> corresponding memory_block.  As find_memory_block() runs in sub-linear
> time this approach is negligibly slower than what we do at present.
> 
> In exchange for this lookup at hot-remove time we no longer need to
> call memory_add_physaddr_to_nid() during drmem_init() for each LMB.
> On powerpc, memory_add_physaddr_to_nid() is a linear search, so this
> spares us an O(n^2) initialization during boot.
> 
> On systems with many LMBs that initialization overhead is palpable and
> disruptive.  For example, on a box with 249854 LMBs we're seeing
> drmem_init() take upwards of 30 seconds to complete:
> 
> [   53.721639] drmem: initializing drmem v2
> [   80.604346] watchdog: BUG: soft lockup - CPU#65 stuck for 23s! [swapper/0:1]
> [   80.604377] Modules linked in:
> [   80.604389] CPU: 65 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc2+ #4
> [   80.604397] NIP:  c0000000000a4980 LR: c0000000000a4940 CTR: 0000000000000000
> [   80.604407] REGS: c0002dbff8493830 TRAP: 0901   Not tainted  (5.6.0-rc2+)
> [   80.604412] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000248  XER: 0000000d
> [   80.604431] CFAR: c0000000000a4a38 IRQMASK: 0
> [   80.604431] GPR00: c0000000000a4940 c0002dbff8493ac0 c000000001904400 c0003cfffffede30
> [   80.604431] GPR04: 0000000000000000 c000000000f4095a 000000000000002f 0000000010000000
> [   80.604431] GPR08: c0000bf7ecdb7fb8 c0000bf7ecc2d3c8 0000000000000008 c00c0002fdfb2001
> [   80.604431] GPR12: 0000000000000000 c00000001e8ec200
> [   80.604477] NIP [c0000000000a4980] hot_add_scn_to_nid+0xa0/0x3e0
> [   80.604486] LR [c0000000000a4940] hot_add_scn_to_nid+0x60/0x3e0
> [   80.604492] Call Trace:
> [   80.604498] [c0002dbff8493ac0] [c0000000000a4940] hot_add_scn_to_nid+0x60/0x3e0 (unreliable)
> [   80.604509] [c0002dbff8493b20] [c000000000087c10] memory_add_physaddr_to_nid+0x20/0x60
> [   80.604521] [c0002dbff8493b40] [c0000000010d4880] drmem_init+0x25c/0x2f0
> [   80.604530] [c0002dbff8493c10] [c000000000010154] do_one_initcall+0x64/0x2c0
> [   80.604540] [c0002dbff8493ce0] [c0000000010c4aa0] kernel_init_freeable+0x2d8/0x3a0
> [   80.604550] [c0002dbff8493db0] [c000000000010824] kernel_init+0x2c/0x148
> [   80.604560] [c0002dbff8493e20] [c00000000000b648] ret_from_kernel_thread+0x5c/0x74
> [   80.604567] Instruction dump:
> [   80.604574] 392918e8 e9490000 e90a000a e92a0000 80ea000c 1d080018 3908ffe8 7d094214
> [   80.604586] 7fa94040 419d00dc e9490010 714a0088 <2faa0008> 409e00ac e9490000 7fbe5040
> [   89.047390] drmem: 249854 LMB(s)
> 
> With a patched kernel on the same machine we're no longer seeing the
> soft lockup.  drmem_init() now completes in negligible time, even when
> the LMB count is large.
> 

Yeah, as long as you remove_memory() in memory block granularity, this
is guaranteed to work.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb