linux-kernel - Re: [PATCH v6 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Fri, 22 Nov 2019 14:29:32 -0500
From:   Waiman Long <longman@...hat.com>
To:     Alex Kogan <alex.kogan@...cle.com>,
        kbuild test robot <lkp@...el.com>
Cc:     kbuild-all@...ts.01.org, linux@...linux.org.uk,
        peterz@...radead.org, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com, rahul.x.yadav@...cle.com
Subject: Re: [PATCH v6 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On 11/22/19 1:28 PM, Alex Kogan wrote:
>
>> On Nov 20, 2019, at 10:16 AM, kbuild test robot <lkp@...el.com> wrote:
>>
>> Hi Alex,
>>
>> Thank you for the patch! Yet something to improve:
>>
>> [auto build test ERROR on linus/master]
>> [also build test ERROR on v5.4-rc8 next-20191120]
>> [if your patch is applied to the wrong git tree, please drop us a note to help
>> improve the system. BTW, we also suggest to use '--base' option to specify the
>> base tree in git format-patch, please see https://urldefense.proofpoint.com/v2/url?u=https-3A__stackoverflow.com_a_37406982&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=BxEt1232ccGlMGDinAB0QAUaTFyl-m5sp4C-crHjpoU&s=OzzQqg4fTDV55X-y4vbnGeXoJaPHSvO_EfrUQnMVRHc&e= ]
>>
>> url:    https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_0day-2Dci_linux_commits_Alex-2DKogan_locking-2Dqspinlock-2DRename-2Dmcs-2Dlock-2Dunlock-2Dmacros-2Dand-2Dmake-2Dthem-2Dmore-2Dgeneric_20191109-2D180535&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=BxEt1232ccGlMGDinAB0QAUaTFyl-m5sp4C-crHjpoU&s=uE7ZeYXOFiu09PUVjnCntEe2rR5x_QxS6dEW9twpfok&e= 
>> base:   https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_pub_scm_linux_kernel_git_torvalds_linux.git&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Hvhk3F4omdCk-GE1PTOm3Kn0A7ApWOZ2aZLTuVxFK4k&m=BxEt1232ccGlMGDinAB0QAUaTFyl-m5sp4C-crHjpoU&s=aAKxuXc_c7OF0ffioQfVsIB6H-4Sd9PYxSM7kurm2ig&e=  0058b0a506e40d9a2c62015fe92eb64a44d78cd9
>> config: i386-randconfig-f003-20191120 (attached as .config)
>> compiler: gcc-7 (Debian 7.4.0-14) 7.4.0
>> reproduce:
>>        # save the attached .config to linux build tree
>>        make ARCH=i386 
>>
>> If you fix the issue, kindly add following tag
>> Reported-by: kbuild test robot <lkp@...el.com>
>>
>> All error/warnings (new ones prefixed by >>):
>>
>>   In file included from include/linux/export.h:42:0,
>>                    from include/linux/linkage.h:7,
>>                    from include/linux/kernel.h:8,
>>                    from include/linux/list.h:9,
>>                    from include/linux/smp.h:12,
>>                    from kernel/locking/qspinlock.c:16:
>>   kernel/locking/qspinlock_cna.h: In function 'cna_init_nodes':
>>>> include/linux/compiler.h:350:38: error: call to '__compiletime_assert_80' declared with attribute error: BUILD_BUG_ON failed: sizeof(struct cna_node) > sizeof(struct qnode)
>>     _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>>                                         ^
>>   include/linux/compiler.h:331:4: note: in definition of macro '__compiletime_assert'
>>       prefix ## suffix();    \
>>       ^~~~~~
>>   include/linux/compiler.h:350:2: note: in expansion of macro '_compiletime_assert'
>>     _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>>     ^~~~~~~~~~~~~~~~~~~
>>   include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
>>    #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
>>                                        ^~~~~~~~~~~~~~~~~~
>>   include/linux/build_bug.h:50:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
>>     BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
>>     ^~~~~~~~~~~~~~~~
>>>> kernel/locking/qspinlock_cna.h:80:2: note: in expansion of macro 'BUILD_BUG_ON'
>>     BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
>>     ^~~~~~~~~~~~
> Consider the following definition of qnode:
>
> struct qnode {
> 	struct mcs_spinlock mcs;
> #if defined(CONFIG_PARAVIRT_SPINLOCKS) || defined(CONFIG_NUMA_AWARE_SPINLOCKS)
> 	long reserved[2];
> #endif
> };
>
> and this is how cna_node is defined:
>
> struct cna_node {
> 	struct mcs_spinlock	mcs;
> 	int			numa_node;
> 	u32			encoded_tail;
> 	u32			pre_scan_result; /* 0, 1, 2 or encoded tail */
> 	u32			intra_count;
> };
>
> Since long is 32 bit on i386, we get the compilation error above.
>
> We can try and squeeze CNA-specific fields into 64 bit on i386 (or any 32bit 
> architecture for that matter). Note that an encoded tail pointer requires up 
> to 24 bits, and we have two of those. We would want different field encodings 
> for 32 vs 64bit architectures, and this all will be quite ugly.
>
> So instead we should probably either change the definition of @reserved in qnode 
> to long long, or perhaps disable CNA on 32bit architectures altogether?
> I would certainly prefer the former, especially as it requires the least amount 
> of code/config changes.
>
> Any objections / thoughts?
>
> Thanks,
> — Alex
>
The easy way out is to restrict NUMA qspinlock to 64-bit only. There
aren't that many 32-bit NUMA systems out there that we have to worry about.

Just add "depends on 64BIT" to the config entry.

Cheers,
Longman