> On Mar 6, 2020, at 7:56 PM, Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote: > > > > On 03/07/2020 06:04 AM, Qian Cai wrote: >> >> >>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> wrote: >>> >>> Hmm, set_pte_at() function is not preferred here for these tests. The idea >>> is to avoid or atleast minimize TLB/cache flushes triggered from these sort >>> of 'static' tests. set_pte_at() is platform provided and could/might trigger >>> these flushes or some other platform specific synchronization stuff. Just >> >> Why is that important for this debugging option? > > Primarily reason is to avoid TLB/cache flush instructions on the system > during these tests that only involve transforming different page table > level entries through helpers. Unless really necessary, why should it > emit any TLB/cache flush instructions ? > >> >>> wondering is there specific reason with respect to the soft lock up problem >>> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ? >> >> Looks at the s390 version of set_pte_at(), it has this comment, >> vmaddr); >> >> /* >> * Certain architectures need to do special things when PTEs >> * within a page table are directly modified. Thus, the following >> * hook is made available. >> */ >> >> I can only guess that powerpc could be the same here. > > This comment is present in multiple platforms while defining set_pte_at(). > Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at() No, barrier() is not enough. > does as compared to WRITE_ONCE() that avoids the soft lock up, just trying > to understand. I surely can spend hours to figure which exact things in set_pte_at() is necessary for pte_clear() not to stuck, and then propose a solution and possible need to retest on multiple arches. I am not sure if that is a good use of my time just to saving a few TLB/cache flush on a debug kernel?
